A lot of people I know say having two cards in SLI is more of a "bragging" point than it is actually logical (for most users anyway).
I have a great desktop still, overpaid a while ago and spent money on an Alienware, but the computer is now almost 5 years old. I am looking to probably build my own soon. I'll probably salvage some from my current desktop (like the DDR3 I have is pretty new, and the hard drives are new too due to an issue I had a few months back).
I'll be waiting a little while though, because I want to wait until some of the better stuff comes out so the prices on current stuff goes down.
'11 BMW E92 ///M3 - ZCP and DCT
'13 Toyota Tundra - 4x4 Platinum CrewMax, 5.7l iForce V8