that’s the question…
I would say… W8 untill ATI is supported and get dual GPU shappgire 4 gb.
If u badly on heat then if u can afford 480 gtx go for gtx, but if u can aford more gtx460 go for 460 ? See how many cored/mghz u get from each gpu, - 20% on each extra gpu of performance and do ur best 
Right now anyways, the answer to that question hinges on how much RAM your scenes need to render. If you can fit your scenes in 1.5Gig, then by all means go for the 480, with substantially more cores.
-Alan
Will ATI even be supported?
I would assume that since they mention full support for OpenCL that is “should” be supported, but you know how that goes. I hope ATi can provide a really solid initial driver for that card to help it gain some traction in the GPU arena. Until I read some really positive reports about it, it will not be on my recommendation list for a while. 
what I find incredible, is that when all the other companies uses widely cuda to program there renderer and Vray looks like the only one that is trying to do it in “opencl”, ATI/AMD doesn’t seems to do much to support more openCL. But, with the final release of VrayRT (and in the mean time with VrayRT Beta) I’m sure that a lot of graphic cards will be sold because of this product…
Stan
Can you wait until 2011?
http://blogs.wsj.com/digits/2010/09/21/NVIDIA-provides-rare-glimpse-at-chip-roadmap/