Medieval 2 is not that modern, really. PC tech moves so fast it's ridiculous. But even the most modern games that are actually multi-threaded would likely run the same on a 3GHZ dual-core as on a 3GHZ quad-core. One core will always take most of the load, so speed of individual cores is IMO more important then more then 2 cores.
If you're looking to upgrade, E8400 or E8500 is a good choice (1333MHZ FSB though). Probably will be my next purchase when I move on from my E6750.