unless they are written natively for Mac
There's not that many programs being written for the Mac these days anyway, certainly not games. I doubt that Civ 5 will be available for the Mac for awhile at any rate. And even high-end productivity software for the PC is being written with multi-cores in mind, just not the stuff being used by the general public. Unless you have a specific need for a high-performance machine, for doing things like video rendering etc. there's very little point in going for 6-8 core processors. Having one right now is good mainly for the bragging rights alone.