First off, the Q6600's rock!

And as far as setting cpu affininity goes, In Octopus under the Profiles configuration dialog you add a new Config.wtf modification named processAffinityMask the values are a bit mask so Processor 1 = 1, Processor 2 = 2, Processor 3 = 4, Processor 4 = 8. If you wanted to set processor 1&2 you would set it to 3. You can't assign affinity to more than two cores. I'll leave figuring out bitmasking up to you, but here's a little hint: It's additive!

A good affinity assignment for 4 shamans on one box is 2, 4, 8, 8, which leaves core 1 100% available to the OS and background tasks.