In practice, the dual-core E2200 should be better than the single-core 3700. You probably even have double the RAM installed in your newer system.
Something else must be the matter. What OS are you using - WinXP / Win7 / other?
I have similar systems to yours & the dual-core setup with double the RAM easily out performs the single-core system. The difference is quite clearly noticeable when multi-tasking. G