How seriously do you game?
The big difference between socket 1156 and socket 1366 when gaming is the PCI-E lanes when you're using multiple graphics cards. Socket 1156 will run its two PCI-E 16x slots at 8x each. How socket 1366 splits its lanes depends on the motherboard, but more often than not you'll get a pair of full 16x for 2 cards (changes to 16x 8x 8x usually for 3).
The point being socket 1366 is better suited long-term for multiple graphics cards. Will socket 1156 only doing a pair of 8x be a problem? No, not for a bit, but it is a limitation to be aware of now if you're planning on sticking with the system for a while.
Yes, socket 1366 uses triple channel so you'll want multiples of 3 up to a maximum (with a few exceptions) of 24GB. Triple-channel gives more memory bandwidth that you can shake a stick at, so that's one less bottleneck to worry about.
At the end of the day, if you're a hardcore gamer, or want the capacity to add multiple graphics cards as cheaper upgrades when the next generation stuff comes out, then go 1366. If you're content to run a pair of Radeon 5870s until your rig is outdated enough to warrant a whole new system, then you can save a little cash and go 1156.
It all comes down to how much graphics power you want, and what you can afford.