New ATI Models and drivers have done away with most if not all of the previous "BAW THEY HATE OPENGL" issues. So really go with whichever.
I'm running a laptop with a Mobility Radeon HD4650 and things seem fine here.
However! There are benefits outside of SL that may persuade you to go NVidia, depending on if you play other games.
PhysX for one, CUDA (although I'd assume OpenCL will eventually catch on for being more widely supported), and 3D Vision (although you'd need an external monitor for that) are all good reasons to go NVidia.
If price is a big factor, go ATI for sure.
-Asami
Edit: Windows 7 is fine also. 64bit here.