The cards will run at the speed they are set at. It is best to get two cards that run at the same speed, but if not try to overclock the slowest card to match the overclocked card.
Ahh, I see. Then what happens with the HD 6790 in the second slot? (It's even less expensive) The 6790 actually has faster clocks than the 6850. Will the 6850 slow down the lesser 6790? Sorry to be so dense.