i dunno crap about ATI
but as for nvidia, any 8 or 9 series with 512 memory, it really don't matter what ones. go by price and not power, they will all give you highest settings.
HOWEVER
if your playing on a screen bigger then 19inches, and require highest resolution use, then you will need to grab one of them 512+ that also clocks well. (core >= 1000mhz, and shader >= 400mhz)