@Katana.

I have a computer with the same specs as yours. I run the game at a slightly lower resolution (1280*1024) with 4*AA and AF. I had only some lag in sieges were the enemy had several stacks (huge units).

I have Kingdoms. There is no mention in the manual or in the readme of Kingdoms of support of dual core. I think we have to wait for empire for support of dual cores.