Cuda/openCL doesn't care if you have onboard SLI, as long as there is an active slot open it just works and the scalability in speed more then doubles in SLI depending on the card. You really have to see MLT(Metropolis Light Transport) in action to appreciate the unbiased physical lighting being calculated. Not to mention, motherboards now come with 10 slots of 16xPCIe, as well as external GPU solutions like a CUBIX (a poorman's quadroplex) at a fraction of the cost.