My 250W PSU is loaded with is a Radeon9600XT with an Athlon XP2000 and 768Megs of memory. It's been working nicely since a couple of years now.
I looked at the X1600Pro AGP, for which ATI recommends to have a 350W PSU in the system.
To add to what Paparazi says, I'm pretty sure that my actual system would still work with an X1600, but if there was an Intel P4 at 3+ gigs I would not bet on it and I'd get a 350W Antec. It depends on how loaded your system is.