SLi must be identical?

[grazhopper]

I feel stupid asking this question but in order to have two SLi cards, do they have to be identical?? If not, what would make them both compatible to one another?

 

[Geoff]

They must be the same model and specs (ex. 2x 6600GT's both with 500/1000Mhz speeds). The brand doesnt need to be the same, but its less likely that there will be a problem and you may get better performance if there identical cards.