Do you actually need to OC them? First see whether you can play the games you want at stock. If it's unplayable on dual 5770s then a slight OC won't change much and a major OC is not worth the risk.
But, if you must OC, don't adjust voltages, and don't go more than +50MHz even if it looks like it can take it. You want complete, guaranteed stability. Also be aware you'll never be able to return it under warranty, not even for a different issue.







