Whenever I build up a machine I follow these guidelines.
1. Case, must be rugged and have plenty of room to move and expand. This computer's case is a CM Stacker, the only thing it lacked was a handle, so I gave it one.
2. Good quality PSU, why spend a million bucks on all the other hardware and skimp on the workhorse? Spend a few extra bucks get something decent and reliable.
http://www.extreme.outervision.com/index.jsp for PSU calculation
3. Decide before hand whether you want dual graphics cards or more for future proofing your machine as much is possible, the hardware in this rig is nearly a year old and still plays everything that has come out before and since with every setting at maximum. Crossfire 4870X2 cards before you ask
4. i7 processors seem to be the best bang for buck at the moment, but they are memory hungry bastards, feed them up well.
5. Solid mainboard with plenty of USB ports and PCI for any older cards required ( Catweasel ) and serial port / parallel port cards.
All I can think of for now.
