No you cant unlock it to ultra, but you can overclock it to faster than ultra speed. Dude don't get SLI. 90% of the time a single 7800gt will beat two 6800gts. If your're going to spend that much money on a video card, get a single 7800gtx 512mb, or get a single 7800gt and get another one when you get the money, but seriously, if you get sli, it will make no difference at all in the 90% of games that arn't optimized for it, and it will only make about a 40% difference in the 10% of games that are. You're paying 100% more for a 40% increase, on a select few games that even use it. And don't get the 512mb 6800, it will just slow it dowm, because the card dosn't have the power to use all that memory. currently the only cards that have 512mb and can actually make use of it are the 7800gtx 512mb and the x1800xt.
so, umm, err yeah