Do you currently have the PSU? If so, it only has 1x 6pin pci-e power connector and the 6870 requires 2 (that can be fixed with adapters but it's not ideal). I believe the 7850 only requires 1 connector and is better (also more expensive) than the 6870. The 6850 also only requires 1. If you're also buying the PSU, then just get the CX500 and the 6870. Hope that helps!
well I have the PSU installed and my friend is giving me his Radeon hd 6870... I bought a logisy's adapter to make it work. I don't think I'm EVER going to max out the card... gonna play starcraft 2 and skyrim mostly.