I've been using nVidia cards in Ubuntu boxes for years. The closed source drivers have been great and I've even found my way around the nvidia-settings program when I've needed to. I'm now in the market for a new graphics card. Is there (at present) any reason to entertain switching to an ATI card?
Does not matter, just check the compatibility list before you buy (both make both good and bad cards).
and also http://free3d.org
any , doesnt really matter!
but IMO i would go buy Nvidia as their drivers are more stable on both windows and linux as compared to ATI, but if u have a hackintosh compatibled hardware and plan to build a hackintosh in future go for ATI...
if u dual boot with windows den ATI seems to be the budget guy's friend,
if u need help with Hackintosh compatible hardware have a look here see if its possible for you http://tonymacx86.blogspot.com/2011/11/building-sandy-bridge-customac-buyers.html
but if u ask me i would go for NVIDIA any day! as game dev support it most