Setting aside specific components, I'll share my general strategy for buying a computer -- I've always been well-served by buying just behind the bell-curve of price/performance. Aside from a few specific components I've picked up when feeling lavish (and generally also when gainfully employed and well-compensated), I've never bought the absolute best-of-the-best. Computer components, like most other goods where a "high-end" market exists, can rather easily cost you 2-3 time more for top-tier components that are only marginally faster (typically 15%, give or take). It really doesn't make sense to buy thosoe unless you're working in a profession where time is money, where such components represent a bottle-neck in your workflow, and where freeing up that (up to) 20% of time will actually pay for the massive different in cost. This is especially true of "professional-grade" components like workstation GPUs or processors, but its even true of consumer-level components.
A good, fast, core i7 can be had for around $300, but you can easily spend $600-$1000 for the very fastest models, or the "extreme" versions -- for that extra $300-$700, you probably get 400Mhz base clock-speed, and a few more megs of cache -- or, maybe you get two more cores (but lower clock-speeds) and the ability to host more memory. GPUs are interesting, in that performance actually scales close-to-linearly in the higher-end range (owing mostly to near-perfect SLI scaling and SLI products that are basically 2x their single-gpu conterparts) but down in the more main-stream ranges its again not difficult to find GPUs that cost half as much as another but give you 75-80 percent of the performance. RAM is different in another way -- you pay dearly for higher-clocked lower-latency modules that make almost no perceptible different in actual workloads -- in any real-world choice scenario, it's pretty much always better to take 2x slower RAM than 1x of much faster RAM.
In this way, you can very easily spend right around $200 per major component (CPU, Motherboard, GPU, RAM) and have a very nice system rounding up to an even $1000 for the rest of the components -- all the better if you have an old case, power-supply, or disk drives you can make use of.