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Between Windows and anti-virus/firewall thats effectively one core used
Well, I wouldnt say it goes as far as using an entire core.... But on average I would guess that the second core sits around 10-15% utilization when running windows + a lightly threaded game. (Yes even older games were threaded.. but generally 99% of older games runtime is a single 3d graphics engine thread).
But thats still cpu time that would be stolen from the graphics thread on a single core system.
The more you multitask, the more benifits you get from multicore even with older applications, and of course the future will be fully threaded graphics engines which make full use of as many cores as you have. It will probably be a while before we see a graphics engine like this, as its a lot of difficult programming.. but I have no doubt it will happen.
I would recommend a dual core over a single core, quad takes a lot more justification. I have just downgraded from a Quad Core @ 3.2 to a dual at 3.6 and in all honesty 3.6 feels a tad faster in the OS. The main bottleneck on the OS is now the disk subsystem with fast processors its the law of diminishing returns now.