Do u mean CPU??
If so that is incorrect. Rendering in things like maya (by default uses the mental ray renderer) is CPU dependant.
Having a good graphics card will help you with the actual amount of on-screen polys and texture etc. Rendering 3D images is a whole different balll game and has to do with the CPU.
So, an i7 build will greatly help you. It will be much better to go with an i7 build with a weaker graphics card, than a core 2 quad build with a strong graphics card. The i7s multithreading also helps quite a bit in rendering.
I use 3d Studio max, Photoshop, Illustrator, some Maya and alot of CAD for my uni and work.
To break it down these are where things will help you:
Rendering (eg. rendering with GI etc.) = A strong CPU will help you.
On-screen display of complex 3D forms (eg. lots of polys) = A stong gaming GPU or workstation card.
Mulitasking and large format, mulit-layer images (eg. photoshop A0 images) = Lots of RAM will help.
I would say get an i7, with 6GBs of ram or more, then look at what kind of graphics card you can get. As for Nvidia vs ATI - its very difficult to find benchmarks for 3d programs on gaming cards - most will only deal with workstation cards. In the workstation world ATI has the upperhand at the moment, but that doesnt mean it translates to gaming cards.