Nah the RAM's not the prob.
That dual core could bottleneck some CPU-intensive games that respond to quad cores, there're only a handful. Most are good up to two cores, and yeah for these games 3 GHz should be fine, even with a 660 Ti.
What resolution are you playing at? I think you should be doing 60 fps maxed out at under 1080p in most games.
If you feel there's a bottleneck, turn off CPU-intensive effects like CPU based physics stuff.
But, if you upgrade later on, the 660 Ti will be a good companion at 1680x1050 or 1600x900. 1080p might be variable.
Also, DDR2 is expensive, no point.