I doubt that data transfers over PCI bus, CPU to GPUs or GPU to GPU, would eat any advantage such system would have. I.e. it will be hardware restricted after all. Anybody have an idea if the PCI-e can handle direct communication between devices or does it all go through north bridge?
The SLI bridge is used to reduce bandwidth constraints and send data between both graphics cards directly. It is possible to run SLI without using the bridge connector on a pair of low-end to mid-range graphics cards (e.g. 7100GS or 6600GT) with NVIDIA's Forceware drivers 80.XX or later. Since these graphics cards do not use as much bandwidth, data can be relayed through just the chipsets on the motherboard. However, if no SLI bridge is used on two high-end graphics cards, the performance suffers severely as the chipset does not have enough bandwidth.
Best option would probably be a card with multiple GPUs.
-C+