ATI recommend a minimum 450w PSU for the X1800/1900 series
I've just upgraded my high-efficiency Antec 430w NeoHE PSU for the 550w version of the same.
By the looks of things, unlike the CPU world graphics cards are going to get even more power hungry in the future.
For a single GPU system, a good 450w PSU would do you fine, but in the interests of futureproofing I'd recommend getting something a bit higher especially if you want to overclock.
Personal recommendation, high efficiency, good reliability, and low noise:
Antec NeoHE 550w Modular PSU