Even if you get the HD 5850 o/c'd to 850MHz core clock 1200MHz memory clock can be reached, but it will not give same performance, because the HD 5850 only has 1440 shaders, and the HD 5870 has 1600. You must also bear in mind that the HD 5870 can be overclocked as well, some can hit over 1GHz on the core clock. I reckon the HD 5850 is enough to play most games maxed out at 1080p, except Crysis, Metro 2033 and Shattered Horizon. If you still feel you need more juice, then get the HD 5870, but right now an HD 5850 can handle DX11 games just fine. If you don't mind getting the HD 5850 now, remember that you can always upgrade by buying another one, and get HD 5970 performance, whereas buying two HD 5870 would be $200 more expensive in total, and many games wouldn't need that power, except the three I mentioned previously, but, Crysis is more of a benchmark than a game imo.