I've been having some problems lately with graphically intense + cpu intense games such as medieval 2 total war and the likes. Everything runs great until a couple hours into and I get random restarts. the whole system just shuts down and reboots. I know its not my power supply, but I'm not sure if its the mother board or OS not working with the SATA hardrive properly. I bought a amd 4200+ about a year ago along with a new mobo and 2gigs or ddr 800 memory. But had an old geforce 7600 gs (AGP) gpu, so the mobo I picked doesn't have pci-e slots. I'm curious on your guys opinions about a new mobo, that would fit my cpu, and memory. but have 1-2 pcie-e 16x slots. and have enough room for big fans. if its a sli board the cards need to be enough apart to keep cool. I like overclocking to get the most money out of things so space and cooling is important to me. I alos got a decent mid tower case in mind that should alow enough air flow from front to back. but I hate my pci slots being way to close to each other and i have a pci sound card to run my surround sound. so haven't looked at sli musch cause of the space issues, but if I buy a new mobo I want it to be upgradeable in the next couple years, gpu wise. the am2 socket should be able to net 3-3.5 ghz dual processor, before it gets old. so basically I'll be keeping the following hardware 550 watt power supply with 2x 12volt rails and pci-e cables (might upgrade to a 700 watt psu, a work buddy might be getting rid of his) amd athlon 64x2 4200+ 2x 1gig oskill ddr2800 1x pci sound card 1x ide hardrive 1-2x sata hardrives so I'm just looking for a good mobo, that has pci-e 16x support, sli or not, I'm aiming at a Geforce8600 or 8800 if the prices will drop after christmas. Money is an issue so I'm looking at spending about 200 dollars, 300 tops. the less the sooner I can get it.

A restart like that is more likely a bad mobo, or bad RAM, than something with the hard drive. It could also be cooling, as the system will shut down if it thinks it's overheating. 550W is quite sufficient for any single-card rig, and for most dual-card rigs, too.

I had a 4200+ X2 and upgraded to a Core 2 Duo E6850 a year ago, and it was a quite noticeable step up. I would suspect your current RAM btw. Get two sticks of some quality value 800 MHz RAM to compare with. And personally, I never overclock, because the additional instability isn't worth it to me, but your mileage may vary.