Currently i5 motherboards have issues with scalability of GPUs. They're fine with a single GPU, but when you add in a second one you start to bottleneck, this is because the CPU has an on-die PCIe controller with 16 lanes.
I've been told (but don't have any links) that future motherboards will have additional PCIe lanes coming off the NB, which of course will be subject to more latency than the onboard PCIe lanes.
If you're doing a multi GPU setup right now and don't want to break the bank I would recommend a high-end Phenom II X4, or if you really want to cheap out a high-end X3, but you might regret that later if games adopt quad-core support widely.