A concern is with the memory. I don't understand this: I run 2 480's and MSI AB shows each cards memory being used, so for example: GPU 1 memory usage: 1400MB, GPU 2 memory usage: 1400MB, so I equal that to 2800MB = 2.8GB, but other forum member informed me that MSI AB is wrong, that only 1 cards memory is being used so it's only 1400MB, and it is not possible for both cards memory to be used because SLI is not designed like that.
So I think if that is true you could get a new 4GB GTX 680 for £500! Then get SLI later, so you have 4GB memory.
If it isn't true, get a standard GTX680 2GB the still go SLI later, so you have 4GB