Many games these days are programmed with a single processor in mind. The extra power of a Dual Core is not optimized for it. The programming languages are not written in a way to accomadate an extra processor.

However more recent games are being made to fully utilize the extra power through parallel programming (same instructions executed at the same time). As games become more and more complex and demand more power, they will need to be written to utilize more power.

I think Unreal 3 engine is optimized to take advantage of dual-core processors