If it's the regular 560 and not the Ti then the only difference between it and a 460 is clock speed. The 560 is the better card, but if there is a big price difference the 460 is close enough and you can overclock it to make up the difference, it will just run a little hotter.
Not sure where you are, but in the UK 460s are starting to go for stupid prices. I just picked up a pair for £100 each. That's nearly 50% cheaper than a straight (non ti) 560, and they overclock to buggery and back. A pair of 460s in SLI cap out somewhere between a GTX570 and GTX580, costing £350 according to the GPU charts on this site. Absolute steal right now, mainly because they won't be around much longer. They support all the current gen stuff, and are great value right now.