I just got a second Sapphire 7870 OC to crossfire and am having issues with crashes. Each card runs fine when they are running by themselves, but when i play any games I crash when in crossfire. I've tested the setup with BF3, Sleeping Dogs, and GTA IV. Here are my specs:
An HX750 should be sufficient to power two 7870s. Did you make sure to download the latest Catalyst Application Profiles pack? If you didn't that can cause problems when trying to run these games in Crossfire.
If that's not the problem you likely either have driver problems, or issues with those particular games. You may want to try using Driversweeper and reinstall the 12.8 drivers, or try an earlier revision like 12.6 or 12.7 beta, and see if that helps.