Agree on the 2 cores for gaming, 4 cores for games and virus/adware scans. Or if you don't mind buying AMD, you can get one of the 4 core processors with a locked core for 3 cores at a lower price than a 4 core.
If you only play single player games, then it would be entire feasible to have two separate accounts. One where on startup it would be networked and have virus/adware applications running, and one where on startup it would have networking disabled and not start virus/adware applications. You might be able to squeeze online games onto two cores, but I'd probably disable virus scanners then and trust a proactive firewall like Comodo.