The market in used graphics cards is weird. I'm going to show you how you can save yourself a bit of money with a 580 instead.
These aren't real prices, just speculations. This is what you'd think:
GTX 580 - 400$ Used [After new generation] - 40$ = 360$
GTX 560 - 200$ Used [After new generation] - 20$ = 180$
But that's wrong. When one GPU in a generation goes down by x amount, the rest tend to decrease by a very similar amount, not a factor of that amount. So if you had two 560s, you'd lose 2x rather than x.
Also, 580 will save you a bit of cash on your electrical bill.
Also, 580 won't have SLI scaling issues.
Also, 580 won't have overclocking issues (SLI overclocking issues aren't that major, but they do exist)
Also, 580 will work in setups that don't support SLI, making building and selling that much more convenient.
Also, 580 will be easier to alternatively cool as you don't have to buy two coolers.
Also, 580 will work in setups that don't have the power connectors two 560s need (four six-pins, I assume?)
Pros for 560: SLI looks cool and if one GPU goes bad, you've got the other to use while you RMA or throw away (lol RMA and throw away rhyme).