I just built a system and am documenting the process on my blog. That's because it was the first one I have ever put together from scratch, though I have certainly replaced graphics cards and hard drives before.
My needs are different from the OPs, in that I require quite a high-end performer that must also be as quiet as possible.
So far I've posted the
introduction and
components overview. Two more parts coming, including three dozen photos showing how to assemble the thing step by step.
The system consists of a Core i7 running at 3.2GHz with 6GB RAM, 1.5TB hard drive, a top-notch power supply, EX58-UD5 motherboard, Scythe CPU cooler, designer case from Sweden, the most powerful passive graphics card possible, Windows 7 Professional 64-bit, etc. ran me 1000 pounds sterling (including tax). I can overclock to 4GHz if I want more heat and noise.
An equivalent Mac would have been twice that.
Everything runs pretty well instantaneously and all 8 cores get used, since the OS itself load balances between processes.
One could get away a lot cheaper if silent running and extreme performance were not priorities.
I added a 300GB drive for the OS and will be putting in two 1TB drives I already own, for close to 4 TB total. One drive I will use as an internal backup, plus I have 2TB external.