Definitely worth updating to GTX 660. That's about a 75% performance increase, and access to another GB of VRAM (important if you're playing current games at 1080p).
Two GTX 460s in SLI is a formidable setup, but newer games have better optimization for the GTX 6xx cards, you open yourself to possible microstuttering and SLI-compatibility issues, and you will limit yourself to 1GB of usable VRAM. Not a big deal as long as you don't mind turning Textures down in games or only using FXAA instead of MSAA.
^ agreed, I have 2 x GTX 460 768 - MB cards in SLI and while it is a nice setup, I am limited to 768 meg VRAM and cannot crank all texture settings at 1080p. I will be upgrading to a similar card such as the 660Ti or even a 670 within a few months.
I would definitely upgrade to the GTX 660 from the 460. Do not do what I did. I had a 560 1gb and wanted more performance and bought a second one to SLI, and then most of my games bottlenecked because of the awfully low amount of Vram. If the performance gain from the card isn't enough, the increase in Vram should be if you are gaming in 1080p. The GTX 670 4gb I have is much better than the GTX 560s because of all the Vram even though the performance is roughly even. Just my 2 cents.