My existing motherboard doesn't do SLI, only crossfire. It was one of the first to support the Q9450 and at that point there were shenanigans between nVidia and intel. Alas, thanks to my X48 chipset, no SLI for me.
Of course, that could be the perfect excuse to get an X58 and core i7...
However, getting a bit off-topic here :-)
I think the 460 only supports two-way SLI, but even without that, can't you address the cards individually with CUDA, or does the API depend on hardware SLI?