In my opinion I think you should switch out of the E6750. That's most likely your biggest bottleneck. Another could possibly be your HDD, pending on whether that's IDE or SATA and whether it's SATA I or SATA II.
Although, it would be helpful if I knew how much ram you were running with now. :)
It will help a LOT at endgame when its juggling so many things at the same time. I play on huge myself and its a pain once you hit the modern age. By that time everyone has met each other and there are a lot of things going on in the background.