I've recently been recommending a single, powerful card over two in SLI. For one thing, you may have micro-stuttering problems
with the 460s; they're right around the problem tier. There's a good economic reason for the decision too: if you get the 460s now, you'll have to get rid of them entirely when you next upgrade, but you can look at the 7870 as an investment for future Xfire. Xfire is too effective and relatively cheap to throw away immediately.