Interesting article with ATI and NVIDIA cards benched against each other to see how many CPU cores you need to run games efficiently.
http://www.bit-tech.net/hardware/cpus/2010/07/05/how-many-cpu-cores-do-games-need/1
http://www.bit-tech.net/hardware/cpus/2010/07/05/how-many-cpu-cores-do-games-need/1
Four CPU cores appears to be plenty, and it's upgrade and multi-task proof. If you do leave your torrent client MSN on or music playing in the background, for example, then the game will still have enough spare resources not to chug.
One important conclusion to take away though is that you don't need a 6-core CPU. They do nothing for your games, so unless you already find a quad-core slow in other applications or your day-to-day PC use, don't upgrade for this reason.