This is a graphics cards chart that shows where cards fall in order of best to worse and compares Nvidia to AMD as to what level the cards compare at. The 560Ti is two levels above what you now have and is it worth almost $500 to do so. Sinse you do have two good performing cards you can wait for the release of the new cards from Nvidia and see what those prices will be and there will most likely be some good price reductions , rebates and/or sales. If you were going with an upgrade now then I would be going with a 570 or 580 in order to see a good enough performance increase. But that's just me and what I would do , your budget might be different from mine. I'm not sure that you would be happy with the performance increase in going from two 460's to two 560Ti's. If you did go with two 560Ti's then you might want to look at this card;
EVGA 012-P3-2066-KR GeForce GTX 560 Ti (Fermi) 448 Cores FTW 1280MB 320-bit GDDR5 PCI Express 2.0 x16 HDCP Ready SLI Support Video Card
$289.99 and a $10 rebate makes it $279.99http://www.newegg.com/Product/Product.aspx?Item=N82E16814130738
I know this is more expensive than what you have listed but it is atually a new release that has 64 more processor cores then a regular 560Ti (384) and is really close in performance to a 570.