I would say the difference between the GTX 460 and 560TI would not be particularly noticeable, although there would be an improvement (the 560 has more shaders and clocks at a higher rate).
I would recommend stepping up to the AMD 6950, with 2GB of dedicated memory, it will really future-proof your machine for a good few years. The best versions of the card are made by MSI and XFX.
Hope this helps