I've built two of them from scrounged up and ebay stuff. Started with Salvation Army boxes and went from there.
Be sure and wear one of those anti-static wrist bands so you don't toast anything. My experience is most of this stuff can take some abuse, i. e. more POed I get the more "mechanic minded" I get. :-laf tsk, tsk, remember "He that is slow to wrath is better than he that taketh a walled city" Proverbs somewhere.
I would advise against putting out the bucks for the top of the line stuff when a notch or two lower will do just fine. Lotsa RAM and good video card. Also keep the mindset that this computer stuff is obsolete tomorrow, so no big deal. Finally, remember moving to 64 bit platform is just around the corner, i. e. MS is finally dragging their butt around to converting all their junk software to the platform others have been ready to use for years. Sounds like a new hardware/software bonanza for somebody special -- ka ching!

Yeah, I know the new 64 bit chips will run the old 32 software -- sure thing.

Do I sound cynical... ..... moi
$500 buck for RAM sure sounds high??? Good luck and let us know how it works out. A parting thought, when I built my boxes the prices of new boxes were better than a grand. But now boxes are so inexpensive and available on ebay, etc. it may not be worth your time and money.