What do you mean exactly by "rendering"?
I'm a CG artist and I know that the GPU is almost never used when calculating the final output by most renderers like V-ray, Mental Ray, Arnold...
Actually the GPU is quite secondary for work purposes, and RAM, CPU, and even HDD speed are more important.
That said, when working with millions of polygons, a good GPU can help a lot during interactive manipulations ( moving in the viewports ). Anything within your budget is probably going to be okay. Sadly I can't remember what is inside the workstations at my workplace. Some GTX560ti, I think. They are probably not costing so much nowdays and they are still okay to work with.
For pure rendering purposes ( actually calculating the final pictures ) and compositing afterward, you may want to improve your RAM, though. Can't say about the CPU as I don't know those.
Bottom line : you GPU is probably not really your bigger concern, but anything will probably do. Try the GTX 5xx series as they are probably cheap nowdays.
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