I'd normally recommend waiting for Fermi to see what your options are, but seeing as Fermi hasn't been anything but a disappointment, the only reasonable upgrades from the GTX260 are the HD5850, HD5870, and the HD5970. The 5850 is perhaps the best option if you don't have a strong power supply. The 5870 is even better and doesn't demand much more power. The 5970 is if you want the best card available today. Other than those, SLI'ing your GTX260 will be the next best thing as long as you have the power to run them. There have been a few of them on eBay running around for $130-$150.http://www.tomshardware.com/reviews/best-gaming-graphics-card,2544-7.html
Take a look at the hierarchy chart. You always want to upgrade at least two tiers.