At those resolutions any mid-range card should be fine.
If you want to follow ATI, a HD 4770 is a nice card mid-range card, although you have the HD 4870 and HD 4890 if you want something more. I'd recommend the HD 4890, it's within the price range ($200) and it will last for quite a while if nothing drastic happens. An alternative is two HD 4770 in crossfire, which cost slightly less and have similar performance, with the caveat of being a two card solution.
I'm afraid I don't know much of nVidia's current options, but with the specified price range you have the GTX 275 ($215) which is performance wise similar to the HD 4890 and has PhysX support like all current nVidia cards.
But again, with resolutions below 1680x1050, any mid-range card (way below your price range) should be fine.
As for RAM, if you want to wait until Win 7 comes out, you might as well buy it now if you really want to. It comes out in about 2 months so I suspect there won't be much of a price change.
Finally, I'd wait for CP5670 as he's the resident expert.
