If you don't need the onboard VGA just use chipsets from AMD 790X (for CrossFire half bandwidth configuration) or 790FX (CrossFire full bandwidth) or 770 (single VGA) with southbridge SB710 or SB750. Those are the basic rules and remember that some MB manufacturers don't support core unlocking (e.g. ECS).
Right now I'm using MSI 770T-C45 (DDR2 version) to unlock mine, or you can look at here (http
/uk.msi.com/news.php?news_no=7) for more info on MSI MB which support core unlocking.