When buying a GPU - you always buy the best single, not a pair of weaker GPUs (I'd count GTX 590 and 6990 as one, getting to two of these is loving handicapped)
Don't SLI new gen top-end cards EVER, you'll bottleneck the stuff out of your PC, NVidia are good at keeping no bottlenecks
But go for a GTX 560 TI, sure may be less performance, but you can get a better buffer on Resolution on non-SLI games, soon you can then add another
the TI will always be better in most situations
GTX 560 TI's arent replacing the 460, the non-TI did
the GTX 460 is like 20 frames less than the 560 on AvP, even when factory overclocked and 1GB it goes 10% worse,
BUY THE 560
But if you play SLI Enabled games, Get SLI GTX 460s 1GB, clock them up nicely, and you can get the performance of a 580.