if your want to spend it all a gtx 460 768mb would be best performance, then down a tier at about 150ish dollars would be any of the 5770, then down at about 100 bucks you'd want to go witha 4850, or gts 250,
all of these card require a pci 6 pin power connector, if you pc doesn't have any 6 pin connectors , then the 9600gt green, as you suggested or the 5570 or the 5670, don't require one and simply can be put into your pc pci-e lane