It should work as the cards have same compute capability.

g

Some people are now reporting that they need to have Sli enabled to detect all their cards (perhaps following the release of Boinc V6.10.18)? It was the opposite situation a few months back - Sli had to be disabled to run GPU tasks on multiple cards.

IF this is the case, then your system would either not detect the additional card under Sli (most likely) or your cards would operate at a reduced performance - neither being good.

However these reports may not be exact, and the configuration may depend on the motherboard in question. Have a look in the manual, to see what combinations it can support (probably best to use the online .pdf manual, as these get updated).

Anyway, I would suggest you only try it with Sli OFF; your 8800GT is CC1.1 (G92 core) and your 260s are 1.3 (G200 core).

Remember to finish any running tasks, and select no new tasks, before you start pulling cards in and out of the system - otherwise they will just fail.