Re: EVGA 260's
Well there is no known max speeds. You gotta test them out, I have both mine running at 625core 1050ram without an issue. I used games to test for artifacts, many people use benchmarks like vantage since it would stress the cards more but I rather know if the games display them since they rarely would stress the cards as much as vantage.
Just got by small steps, much like ocing a cpu. No big jumps.
If anything, try to get the low card to match the faster card, since SLi forces both cards to run at the same speed.