how do you rate 6950 vs gtx 570 for driver support and general pixel pushing?, ive heard gtx 560ti not big increase to warrent spending the cash.
Nvidia tends to do better in drivers, but it's really in multi-GPU support where you see why. I honestly would add a second GTX 460 is I were you. The jump in performance from a GTX 460 to a GTX 560 Ti is decent at best:http://www.anandtech.com/bench/Product/313?vs=330
A second GTX 460 will be you good.
EDIT: No SLI or OCing for you huh? Well, I guess an unlockable HD 6950 (some are able to do this with just one flip of a switch) is your best option.