well, each individual card is different. My single 8800gtx is 576/1350/900 stock (that's core clock/shader clock/memory clock). I can overclock it successfully to 625/1430/1010. Some people have had much better luck, many have had about the same.
You really have to test around with your own setup. Having two of these heat producing monsters in your rig will probably lower your oc ability slightly unless you turn to some pretty intense cooling solutions.
Just spend an evening experimenting. Incrementally increase clock settings until you start getting artifacts (weird visual anomalies, usually square patternt) at which point you slightly decrease speeds and your set!