$1500 Gaming build advice

Yes, especially at higher resolutions. if you're gonna be gaming at 1600X900 or lower or indeed you don't wanna play at the highest settings, keep the 6970.

Will also depend on the games you play. Skyrim, SC, WoW, FIFA, NBA, MK etc. won't even require the 6970. The 6950/560ti will be plenty alright.To crossfire the 6970 you won't even need a 950 watt unit. In fact even the 580 can be SLI'ed with a quality 850 watt unit (Source: Guru3d), but for a high end system, it's always advisable to get a bit of headroom to run a cooler temps overall, especially when running OC'ed CPU, RAM AND GPU so as not to over stress the PSU.Yes, true for SB. You may run it OC'ed at 1866 by manually setting the speed in the BIOS maybe directly or by changing the FSB : DRAM ratio if your mobo supports it. Apparently, this Gigabyte board as you say, will support it.I have no idea 'bout the 922. I know for sure that it's got a lot of satisfied customers all over. I recommended it because you seem to like the HAF's. I cannot see any reason for it not to support that build whatsoever.

I personally use the similarly priced Corsair 400R for my builds at this price range. I can vouch for that one and also the Antec 900, the one I used to use.

This is a Corsair 400R GTX 580 SLI build with the Corsair H100-
Depends on high high you wanna go with the overclock. As I said, for moderate clocks, even the 25 dollar Hyper 212+ should be pretty good. At the H-60 prices, you can get the Noctua NH-D14 HSF, if you want to avoid H2O.

Here's the Anandtech review - Corsair Hydro Series: H60, H80 and H100 Reviewed

i5-2500K is just about the perfect gaming CPU for a $1000-$1500 build. It will perform just 'bout as well as the 2600k. It will by no manner or means bottleneck a 580 SLI rig.

But at your price range, the 2600K since you can afford it, is the one to go with. Bragging rights never hurt, eh?  

EDIT: I didn't recommend you an i5 originally. I had recommended a 50 dollar cheaper and 100Mhz slower i7 - 2600 instead of the 2700. The I5's are 23/24/2500(K).

Since you're OC'ing anyways, won't make any difference, unless you're looking to compete in some extreme OC'ing contest, where the better chip will come in handy to go up higher and absorb more voltage