Graphics cards do not matter at all when rendering in Poser/Vue/Max/Carrara. At least, not as long as the rendering engine is CPU based - you just need a fast CPU.
Graphics cards do mantter in preview - the faster the card,, the more responsive your work area is, even at higher quality previews.

The RT2 rendering engine uses the graphics card instead of the CPU for rendering, and is amazingly fast (rendering on a modest nVidia 6600GT was ten times as fast as on a dual core Pentium D). RT2 is not usable from low end or midrange 3D products, but Maya and Max can use it.
I fervently hope that the Gelato project (non-realtime rendering on nVidia graphics cards, 8xxx or higher series) takes off, the graphics card is just far better suited for rendering than the GPU. Actually, the new nVida GTX 280 chip can do about 10 times as many calculations as the fastest Intel quad core CPU, and the newest ATI chip (4870) is even faster...

Still, I'd go with nVidia, since nVidia is far ahead of ATI when it comes to utilizing the graphics card for non-graphics usage. I expect it won't take too long before you won't use render farms anymore, instead, a single PC with a relatively low powered CPU and 4 fast nVidia graphics card in SLI mode will be faster than a current render farm consisting of 32 high end CPUs...

And another thing: nVidia supports XP 64 bit. ATI doesn't.

What graphics card did I choose?
I have 3 fast systems (Q6600/8 GB RAM), 2 of which are dual monitor graphics workstations. One has a nVidia 8800 GT, the other one (somehwhat older) has a nVidia 7800GTX. The third workstation, which I use for programming, rendering and Internet communications, has an nVidia 7900GS.

My server runs on an Athlon64x2 3800+, and has a 8600GT graphics card. Overkill for a server, but I just had that graphics card laying around, and it's slower than the 7900GS.

The pen is mightier than the sword. But if you literally want to have some impact, use a typewriter

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