Second Life software prefers Nvidia cards to Ati. There is a history of SL graphical issues with some Ati cards.
When I specced up my latest pc, the two shortlisted cards were Ati 5770 & Nvidia GTX275.
(Tests show them to be equally good, with Nvidia just having the edge, in some applications).
Because I use SL a lot, I went for GTX275 and have not been disappointed.
With my setup, I have been regularly getting 140-150fps on Emerald viewer.
Must admit, I did have some problems with LL viewer2 and 1.23 though, but V2 is pants anyway.