if you don't really care about DX11 going for 5770 is not bad either. basically in dx9/10 games the pure performance is on par with 4870. but if you want to play dx11 with nice eye candy and frames rates don't go lower than 5850
I agree with all the angles mentioned on why NOT to buy the 4870. Its going on two years old. Driver optimization focus will always be on the newer hardware. Idle power management is much better with the 40nm gpu's. With a huge market of 5770's its easier to find a bargain price than the 4870 right now. So unless its a private sale or a lucky find, its hard to not recommend 5770.