I have 2 BIOS Flashed OC'd 8800GT in SLI, but I'm running a 1000W P/S, I'm not too sure about your success with a 610W P/S even if it is PCP&C, what is your 12v rail amperage, and is it a single 12v rail.
Also I'm running aftermarket Zalman GPU coolers, which dropped my idle temps to 41c, keeping them much cooler than the stock coolers did, what are your idle temps?
I used Riva Tuner to find my OCs, then BIOS flashed the cards to run the OC 726 core/ 1819 shader clock/ 1003 memory, as stock settings, they've been running like that for about 8 months now.
Both cards individually will OC higher than the settings they're BIOS flashed to, but to stably run them in SLI a lower setting than their max is best.
You would need to use Riva Tuner to test your cards, and its imperative to keep them cool, but I still have doubts about the P/Ss handling of the OCs.