My current home build is this (note it was designed to be both a software development machine and a gaming rig):

CPU: i5 2500k @3.7GHz

RAM: 4GB

GPU: HD 6950 (2GB)

HDD: 500GB + 500GB + 120GB (SSD) + 2TB (external)

PSU: 750W

dual 21" monitors

Everything except the extra monitor and the SSD is from early 2011, in fact I upgraded it to 12GB memory at some point but the 2x4GB sticks were dead on arrival, so was the replacement, and I found I rarely ran out of memory anyway, so I just gave up on adding more memory. I should probably give it another shot, but I have neither the time nor the motivation to do it right now.

It still performs excellently in everything I throw at it, so I haven't found the need to upgrade (except the memory, as previously stated). Next candidate for upgrade would be the graphics card, but it can probably go for another year or two before it really starts to hold me back. If I had the money I would probably go ahead and upgrade to the latest and greatest every six months, but I don't, so finding a balance between cost and estimated lifespan is important.

Depends, for my X-Plane/FSX setup on Ubuntu I bought(copied from my avsim profile):

Processor
Intel Core i7 4770K
Processor Cores
4
Processor Speed
ovc to 4.6 Ghz
RAM
16 GB (Corsair Vengeance Pro Red 2x 8GB, DDR3-2400)
Video Board Manufacturer
Nvidia
Video Board Model
Asus GTX-770 DirectCU II 2GB GDDR5
Video Memory
2 GB
Mother Board
Asus Z87 PRO
Hard Drives
Samsung SSD 840 Basic, TLC, 500GB and 1TB HD
Monitor
LG L225WT
Operating System
Ubuntu etc.
Joystick
CH
Yoke
CH
Rudder Pedals
CH
Other
Corsair H90
Asus DVD Burner
Logitech cordless usb MK260 and QuickCam® Sphere™

Nvidia is better supported on Linux than AMD, more SSD as I've FSX/X-Plane on that as well. It depends what you want to do. I've sold my old PC with a coreduo and GTX460 which was good for any games but FSX/X-Plane.
I use for video editing Kdenlive and recordmydesktop.

I use a 15-inch Macbook Pro (i7 2.3Ghz, 16GB ram, 512GB SSD, GeForce GT750M) for all of my development these days. Powerful and very portable. If the game I am making does not run at these specs I know I am clearly doing something wrong. At my home office I also use a 24" monitor for extra screen space.

The portability really was the number one requirement for me since I want to be able to work anywhere. The machine only weights couple of kilograms and has a good battery life. Also the Retina screen is nice for programming because it makes the code look so crispy. :)

The downsides are that is expensive and it has basically zero upgradeability (the battery is glued in, the memory is soldered to the motherboard etc). There are also some sacrifices made to make it slimmer: no optical drive and no ethernet port, so it is WIFI only (there is an adapter available to connect the ethernet to a thunderbolt though). I am ok with these though and I actually think it is good to not keep the all the legacy technologies around.

People only have "a" system? Gads, I couldn't get by without several (nVidia, AMD, Intel graphics, i386, amd64 and ARM CPUs, small screens, big screens, touchscreens, 96 DPI and 300 DPI displays). I have laptops, desktop, phones, and tablets (and a couple of hybrid convertibles). Single-monitor and multi-monitor configs.

I'd love to have a i7/gtx770/16GB/SSD system though. Drool. I've already blown my budget getting the stuff I actually need, as opposed to want. Maybe if I sold some of my children....

Home (primary): i7 4770K, 16GB RAM, Radeon HD7870, 256GB Samsung 840 Pro SSD, 1TB HDD, Win8.1, 23" LG monitor (nice 27" Asus PB278Q on the way, but they're punishing me for choosing free shipping so it'll be another week or so).

Home (secondary): AMD Phenom II x4 "Deneb" (955 Black Edition? Something like that), 8GB RAM, Radeon HD5770, 1TB HDD, Win7, currently controlling my TV as I'm short on monitors and it's better than paying for several hundred trash cable channels.

Home (tertiary): Asus A53S laptop, i5@2.3Ghz, 6GB RAM, GT540M, Win7. CPU hits 87C trying to run Minecraft, which amuses me more than it should. Gets the job done when I can't be at one of my desktops instead.

Work: A refurbished Dell with two 24" HP monitors. Never really bothered to look into its specs much; compiles the business software I work on just as fast as some of the other guys' newer, significantly (stupidly) more expensive (overpriced) hardware, -and- I don't have to deal with Parallels and/or Bootcamp like they do, so I'm happy.

I'm currently using a 7-year old desktop, with a single 23" monitor (1680x1050), running 32 bit Win7 Home Premium, 3GB RAM, a Radeon 4670 with 512MB VRAM (i.e. nothing special).

The most "intensive" software I use (aside from the occasional gaming spree) is Paint Shop Pro XI, when dealing with hi-res images and dozens of layers I run out of RAM and everything lags.

The most-effective performance increases I'd like would be a SSD to speed up compile times, and a second monitor - but I'm making due for now until finances get less tight.

People only have "a" system? Gads, I couldn't get by without several (nVidia, AMD, Intel graphics, i386, amd64 and ARM CPUs, small screens, big screens, touchscreens, 96 DPI and 300 DPI displays). I have laptops, desktop, phones, and tablets (and a couple of hybrid convertibles). Single-monitor and multi-monitor configs.

People only have "a" system? Gads, I couldn't get by without several (nVidia, AMD, Intel graphics, i386, amd64 and ARM CPUs, small screens, big screens, touchscreens, 96 DPI and 300 DPI displays). I have laptops, desktop, phones, and tablets (and a couple of hybrid convertibles). Single-monitor and multi-monitor configs.

And you do gamedev using all of them at once, writing code on your phone? :-P ;-)

yeah, I have multiple devices as well...

don't necessarily use all at once, and probably spend the vast majority of my time at my desktop.

otherwise, trying to run Windows XP in an x86 emulator (apparently an Android port of Bochs) on an Android tablet gives impressive performance.

so totally doesn't take between many seconds and several minutes to respond to user-input... ("System Properties" lists its speed as 6MHz).

a person could then try running Visual Studio or similar on it.

or such...

I'm kinda embarrassed to admit my specs here. Lately, I haven't had any time to make the upgrades to my current machines. My 2nd machine was a fixer-uper and I bought it for $200, and ended up fixing it for $10. When I get a chance, I'd like to upgrade the firmware to 2,1, max out the ram @ 32GB and upgrade the processors to 2x4 3.0Ghz Xeon processors. Contrary to common belief, Macs (not the latest one) are upgradeable.

Machine #1:

Macbook Pro 15" (late 2008, early 2009 edition), OSX 10.7 (Lion), 2.93Ghz Core 2 Duo, 4GB Ram, 250GB HDD, NVIDIA GeForce 9600 GT.

Machine #2:

Mac Pro (1,1 2006 edition), OSX 10.9.2 (Mavericks), 2.6Ghz 2x2 Xeon, 6GB Ram, 80+250GB HDD, NVIDIA GeForce GTX 760 1.5GB

Machine #3:

Windows 7/8 Dual boot, 2.3Ghz? (I forgot) i5, 12GB Ram, ? TB (I forgot again), NVIDIA GeForce GTX 460 2GB? (yes, I forgot... again).

I forgot because my PC is in my office, one that I rarely get a chance to go to anymore because my day career takes up nearly all of my time.

Shogun.

RAID redundancy should almost always be used on the central servers, so that if a HDD fails, you don't lose any data.
GIT is nice because every single user actually has the entire repository on their own machines -- so if you did somehow lose the central server, you can recreate it from the most up-to-date user.

Despite this, I've actually worked for a developer who lost their SVN server once -- 3 out of 5 HDDs in a RAID system failed, causing it to become unrecoverable... We had to reconstruct the server from the data on everyone's PCs, but lost the version history of all files, which actually really hurt things a lot as we were in the middle of tweaking many different systems at the time.