sli with different speeds

[rize runner]

this might have been asked a couple of times already so please don't flame me, i just can't find a straight answer.... i just got the evga 8800gt SC and i love it, all my games are so beautiful looking... i want to get another one but i see some cheaper ones, like the dell deal that just passed.... so cutting to the chase, can i mix 2 different 8800gt that have different core speeds? i know the evga SC runs @ 650mhz, can i mix in a XFX 8800gt that runs @ 8800mhz?

thanks,