Also, getting a great brand PSU will make sure your system is stable, look at brands like Antec, Corsair, PC Power & Cooling. Look for 550W or higher that will allow you to use Crossfire (ATI multi GPU solution). Look at amps on the 12v rail.
Getting a case with plenty of room will make building your system easy, my P150 while perfectly big enough, a little more room would have made it easier to work with. Look at Antec Three Hundred, review here.