The problem is the CPU itself not the amount of RAM. The CPU is pretty old, most modern games take advantage of Quad Core CPUs. In most cases you'll get bottlenecked by the CPU especially in CPU intensive games such as Skyrim, WOW and any RPG games in general. And if you wanted to play online on multiplayer this CPU will be a major disappointment.
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.
I Have that exact setup.... E8400@3.8GHz, MSI PE 660TI. I picked up an addition 4GB DDR2 1066 ram used and it made no difference as 4GB seems to be plenty. The processor is the weak link but I have no problems getting 60fps @ 1080P with High settings. You should be able to overclock the E8400 to 3.6GHz with no added voltage and I would recommend doing that.