Video card Question!

Ok, so here's the deal. I built a new computer a couple of months ago and I built with one 9600 gt 512mb. A friend of mine was getting new cards for christmas so i borrowed one of his 8600gt 512mb overclocked, and sli'd it with my system. Everything runs great got all of the drivers for it, but the games crash after a while and a box pops up and says the driver has failed. So is that being caused by the new card?, or am i not mixing them correctly? do i not have the correct drivers?, everything worked perfectly no crashes with just the one card. My other question is would two 8600gt 512mb overclocked be better than one 9600gt 512mb?

Just to clarify I'm running the 9600gt and 8600gt at the same time...

system specs::
core 2 duo 3.16 ghz
evga 123-750i-ftw
4gb 1066 ram
(cards in question)
audigy SE

Thanks,
Nick

yeah, i was afraid of that...i really could have sworn that i saw an article a while back that you can mix cards, but i had this gut feeling that you cant, oh well worth the try, and another 9600 was already in next months budget lol. It was a good experiment i guess.

Thanks Dirtmountain!