I dont no for sure but i think there is not much between nvidia and ati. So imo the next thing to consider is silence. I hav'nt followed the PC graphics card war as closly as the laptop gpu war so i am a bit in the grey with the latest stuff. But for you budget you are looking at slightly dated stuff to the best of my knowledge sapphire are the only people who make cards without fans. Mayeb there is some more detail in this thread:
http://forums.silentpcreview.com/vie...09fbb6855d66d8
However, you must be warned that the newer graphics cards have there own power connectors, and in turn you may need a new power supply to feed it.
Also worth note is that the increased heat output from a new graphics card could mean that you CPU will no longer run without a fan (as a results of higher in-case tempertures). So in turn you might need to add a case fan to remove the hot air from the case.