The question is why you want or need SLI.
I was intending to get the GTX590 with 1024 CUDA cores, but have read here and elsewhere that Vegas Pro 11 does not max out/take full advantage of all the available CUDA cores as there are other components in your system that have to keep up such as disk i/o, system bus speeds and RAM speed (not video RAM) etc.
I ended up settling on a single ASUS GTX570. It’s a massive beast of a card that takes up 2.5 slots, is about 8 inches long and needs a minimum of 650W PSU with a 4 pin AND and 6 pin power connector. This puppy takes up serious space and power, and a SLI setup for me would entail a PSU, motherboard and case upgrade to accommodate 2 cards. Yikes!
I am not convinced that an SLI setup would deliver the performance benefits, although I have heard of some cow users using GTX460’s in an SLI setup. I think SLI setups of lower end/early model cards was necessary to realise performance benefits, but with the newer GTX5-series cards sporting 384, 512 cores etc, SLI may be unnecessary.
My system specs: Intel i7 970, 12GB RAM, ASUS P6T, Vegas Pro 10e (x32/x64), Windows 7 x64 Ultimate, Vegas Production Assistant 1.0, VASST Ultimate S Pro 4.1, Neat Video Pro 2.6