Rendering isnt helped by GPUs at all.
Mental ray has some OpenGL assist on shadows though I think, but I suspect the difference is very minimal.
Unless you´re refering to viewport rendering, Geforce GPUs seems to do well enough there.
Its a bit unmotivated to shell out the money for a Quadro really.
for rendering performance, a nice quad, I think I saw the Core i7 performing ALOT better in rendering then C2Q CPUs.
And sufficient RAM, 4Gb should be plenty for most type of scenes, thats sort of up to what kind of scenes you work with.
Pretty easy to gauge though, just fire up a scene and watch memory usage I guess. add RAM as needed.
Rendering benchmarks on Core i7
weather the GTX series have any viewport issues might be good to know forhand though.
On XP, I had some really wierd viewport Issues in Maya with my 8800GT.
Not on Vista though.
The 8800 series also have severe performance issues in Multidisplay modes on XP in MotionBuilder, work around that by using Span Mode... Dual View works on Vista though.
If the GTX series has any similar issues, maybe someone who has one can provide some input on that.