The higher the res the better it looks obviously, but demanding more GPU VRAM. With a 6950 2GB you can easily play on 1920x1080 res or higher, if you setlle for a cheaper GPU ( 560 or 6870 ) 1080 will be the max. But again, i would wait if i were you, the 9800 ain't that bad. Maybe buy a better psu now and get a 7000 series when they come out.
Make sure your monitor can handle the high res, otherwise you buy a expensive card for nothing.