Depends on what you are going to put into the computer.
How big a PSU depends largely on what graphics card and CPU you put in there.
Graphics cards can draw anywhere from 10 watts for motherboard integrated graphics all the way up to 289 watts for an nVidia GTX295. And multiple video-card setups (nVidia SLI or ATI Crossfire) will require more power still.
CPUs can draw anywhere from 45 watts for a Celeron DC all the way up to 120 watts for a Core 2 Quad.
So figure out what graphics card, CPU, and other things you are going to put into your machine, and you will be able to determine what power supply you should get.
Oh, BTW, since the Asus P5Q is an ATI Crossfire motherboard, it would make no sense to put an nVidia video card on it, because you will be paying a lot of money for that extra Crossfire graphics card slot which you can't use. Crossfire works only with ATI video cards.
Hope this helps.