The 660Ti has a greater amount of cores than the 660 and is built on a cut down version of the GTX 680 and 670. The 660Ti will provide greater performance in most cases. I agree with slh28, if you play games at 1920x1080, the 3GB is a waste of money. The 3GB will come in handy @2560x1440 and maybe 1920x1200 (to maintain higher settings).
I think it is worth waiting a month or so man, the 780 was just announced at a Geforce competition in China and it will officially be revealed on May 23rd. The 770 will follow with a May 31st release. Finally, the 760Ti (what you would want), will be released in early June (around the Intel Haswell launch). If the 760Ti is just a weak upgrade over the 660Ti, then you can get the 660Ti for cheap, and maybe even step up to a 670 or 680.