As far as the game is concerned, having 2 cores all to itself is ideal (there are a few other threads like for the music and stuff, but nothing intensive). That's only if you're playing single player or being the multiplayer host, multilplayer clients don't run the AI thread so they only need the one core.
The complicating factor is that your computer will always be doing something else too, particularly if you have an anti-virus program that likes to spend lots of cpu scanning stuff. For that reason, having a quad core (or even tri-core) can help give you more
consistent performance even if you could have gotten a dual-core that was (say) 0.75GHz faster.
That said, there's definitely no significant need to get more than 4 cores for the purposes of running this game and not much else at the same time; if you really intend to keep your process list clean during play (which can take a fair bit of time on your part just messing with the system) then a dual-core will probably be somewhat faster if both are the same price.
Welcome to the forums
