I would reccomend NVIDIA but they are very similar.
Nvidia 9800GX2 Nvidia GTX 295 Nvidia GTX 285 ATi 4870X2 ATi 4890
that depends on the model. There are several bad cards but mostly good cards in both brands.
nvidia 8200 gpu score 4.7 ati 3100 is a 3.6
the hd3450
nVidia GeForce cards are very much top of the line cards, as far as whether or not they are the best cards available depends on your requirements. nVidia is a great choice for Gaming, while ATI might be better for media work, such as video encoding.
For Nvidia Cards is Scalable Link Interface(SLI)For ATI its CrossFireX.
ATI is a very respectable card producer, and they have excellent cards to choose from, but nVidia has numerous features and tweaks that ATI does not, and a considerable lineup to choose from as well. I would choose nVidia over ATI.
The NVidia is faster.
nVidia and ATi both support valve games.
Currently, for the PC industry, nVidia and ATi (now bought by AMD) are competing for the top spot of graphics card makers. Coming at a distant third is Intel, who currently only focuses on lower quality integrated chips. As for nVidia and ATi, both sides have their respective strengths. nVidia cards are widely used for many professional settings, such as physics engines and for engineering CAD uses. ATi cards are generally renown for their attractive price and good video decoding abilities. On the whole, nVidia has a slight edge and its affiliations with other companies such as Sony help give it better brand recognition than ATi.
Drivers for Intel cards are already bundled into the X server. Drivers for NVIDIA or ATI cards can be downloaded from the company's respective websites.
The minimum requirements for graphics cards to run Second Life using windows xp are : NVIDIA GeForce 6600 or better ATI Radeon 8500, 9250 or better Intel 945 chipset For windows vista and windows 7 the following are required : NVIDIA GeForce 6600 or better ATI Radeon 9500 or better Intel 945 chipset Linden Lab recommends more powerful graphics cards to get the best experience out of the Second Life grid.