Just get a quad core. Forget the 6 cores as it isn't very useful in gaming. In fact, few games fully utilize 4 cores and the quad core i7's actually show up as 8 cores due to hyperthreading.
To be honest, a 920 running at 2.6GHz should run any game at full speed. The i7 architecture is good and if that game takes advantage of its dual core (or more in some newer games) processing it should run fine at 2.6GHz. However, it can lag if you leave other programs open while gaming or are playing very AI or physics intense games (especially sandbox style games like Garry's Mod). At 3.2GHz (i7 960 stock but attainable with overclocking on other chips) it should run a bit quicker when multitasking but again you probably won't notice a difference in games except for ones that are hard on the CPU. At 4GHz or above, it should run any game for the next few years without bottlenecking the GPU.
Don't waste money on the 6 core chips, they are only useful if you do a lot of multitasking. Instead, save money by going with a 930, overclocking it to 4GHz, and spend the rest on a good graphics card, fancy SSD, or some games.
Remember you can overclock your graphics card as well, if you're going with the HD5970 you'll probably want to overclock them because they come underclocked (from the 5870 speeds) by default. You can again save money by not buying factory overclocked editions, ATi makes it simple to overclock graphics cards with ATi Overdrive, a software utility built into the graphics drivers that has sliders to control core and memory speeds as well as fan speeds. Depending on how far you want to go, you could probably push the stock ones above the factory overclock levels.
