i7-860 (1156) 2.80 Ghz Or i7-920 (1366) 2.66GHz?

If you're primarily gaming on the system, you'll want to go with the 860 - it's a little faster than the 920, but not by much, but would end up being less expensive to build with. If you're also doing lots of video editing in HD, the 920 with triple-channel memory would end up being the faster of the two machines in that arena. The 920 system does use more power than the 860 does, but not much more (maybe 20-50 watts under load). Where a 920 based system can become much, much more powerful is in your choice of motherboard - several newer motherboards come with 4 PCI-E x16 slots these days, which can be populated with 4 video cards or a video card and 3 GPU processing units (Tesla, for example).