I may be preaching to the choir here but just in case

Case and power supply are fairly important. Don't just get any generic stuff.
So compusa, circuitcity, bestbuy, fry's...
Antec makes good cases, sure you can get a cheap case for $40 or less but space inside could be limited or even worse some things just may not fit.
Don't skimp on the case, it'll outlive most if not everything else in your system. Good ventilation, 120mm fans front and rear should be adequate.
Power Supply:
compusa: go with Antec TruePower
circuitcity: Antec (compusa has better prices it seems)
bestbuy: Antec
fry's: Dunno what they have, antec, enermax, fortron, allied true, and maybe enlight good brands for power supplies.