Gpu Upgrade Help

[Olympietic]

      Hey guys, I am currently running a Nvidia GTX 560 (not the Ti version) and I feel it is not performing at the level at which I wan't; thus I feel I need to upgrade. Just to give some computer specs I'm running an AMD Phenom II X4 840 (3.2 GHz) and 16gb of ram. I would like to upgrade by the end of summer, at which time I would expect to have built up anywhere from 300-400 US dollars. I would prefer running with an Nvidia GPU. I have been thinking of a 660 or 670 depending on my money, but I would like some outside input. I'm running 1920x1080 monitor, so no high-resolution performance needed. I don't know how my older hardware (my cpu) would affect it, and this is one of my big questions. Anyways, any input would be greatly appreciated. :)

Edit: I also believe that I have enough power.

[TechFan@ic]

The latest rumors suggest that the 700 series from Nvidia is going to be released sometime next month, so I would wait and see.