You get the biggest bang for the buck with this card right now. I compare GFLOPS between cards but if money is no option, then of course the 5970 is the way to go. Sorry about the formatting, I can't seem to figure how to get all of the numbers in a row.
Card GFLOPS Cost GFLOPS/$ Cost per GFLOP
HD 5850 2088 $189.00 11.05 $0.09
HD 5830 1792 $165.00 10.86 $0.09
HD 5870 2720 $269.00 10.11 $0.10
HD 5970 4640 $700.00 6.63 $0.15