GPU accelerated Physx is rare and mostly insignificant. Its been around a long while and only 2-3 games are really noticeably superior with it enabled.
If you want better performance without spending a lot there are two options I would recommend. Another GTX 260 for SLI or an HD5850. I'd go with the HD5850 if you can afford it. At stock it is a bit better than the GTX 285 but it overclocks crazy amounts. The card allows for the core voltage to be raised which has enabled people to usually get speeds between 1000-1100mhz. That's a 40-50% OC which is incredible and easily better than 2 GTX 260s. It will also be DX11 compatible, use much less power, give off much less heat, have no SLI scaling/compatibility issues, leave a slot free for a future upgrade, support 3 monitors, ect.