When you decide betwixt 5850 and 5870, bear in mind that they overclock to about the same level, and when at equal clocks, the difference in performance is that the 5870 is 5-10% faster. By that I mean it's never more than 10%. So, at equal clocks, if on the 5850 you get 50 fps, then on the 5870, you get 55fps ... in the best of circumstances.
Then look at the % cost increase to buy a 5870, and calculate whether or not this gain is worth it to you.