With that power supply I wouldn't recommend anything over a Radeon 5670 or so. They are about the fastest you can get that don't require an external power connector. The radeon 6670 is slightly faster, but about $25-$30 more and hovers right at $100.

Generally try to avoid nvidia cards now a days if you are power supply constrained as they use more than the radeons of equivalent speed. Personally I got a radeon 5770 a few months ago to use with my 450 watt power supply as I didn't have the cash to buy a new one and the GTX 460 would have pushed me almost to the edge of what the PS can handle.

Either way, it would be a huge upgrade over what you have now, especially if you have been 3 boxing with that system. The CPU should be fine as even my crappy dual core 1.6 ghz amd laptop with radeon 3200 onboard can run 2 copies of the current client with the background fps set low on the second window.