The most powerful card I can think of with the certainty of running on your system is the GT 240 (300W requirement). Its between the 9600 GT and HD 4670 in performance. You may be able to run an HD 5670 on it (400W requirement), but make sure you can easily return it if it doesn't work out.
What brand is the PSU?
Even if it's not very good you should be able to use any card that doesn't have an external power connector. The two best cards without one are the HD5670 and the newer low power version of the 9800GT.
If it is a quality brand you can probably use something better like an HD5750 or HD4770.
Yeah, you probably can get it up that high but only with special software designed to max components as far as they will go. You can see it here; http://www.techpowerup.com/reviews/MSI/HD_5770_HAWK/28....
Peak 80w under normal usage and they got it up to 105w with Furmark.
As for that xbit link the system they get up to 228w already has a video card(8800GTX) which uses considerably more power than the HD5770.
Here's a comparison from last August between CUDA and ATI Stream. The GPUs used are an HD 4770 and a 9800 GTX+. The conclusion said that ATI Stream was a little faster in the tested applications while CUDA produced the better quality images.
Thanks for that. The answer is pretty clear: must wait until nVidia updates their cards, and a few months more until the drivers stabilize. It's good to know that my PSU can handle a decent graphics card.