EDIT: I'd get one of the Gigabyte mATX motherboards that also supports SLI for the same price, but that one should do. I haven't seen a thorough review of that ASUS, so I can't tell you how it will OC--not that the low end Gigabytes are gonna OC that high. Based of the microstuttering review above, you probably want to either go with a GeForce or a higher end Radeon for dual card configurations.
I'm mostly sure you could Crossfire a 6970 with that PSU. That said, I think a 6950 is plenty good and worth the price savings. If you're open to SLI motherboards, then the GTX 560Ti gets as low as $175 (keep an eye on slickdeals and get on Newegg's email list).
The bottom line is: if you're building a gaming rig, then the graphics card should come first. If it's a multipurpose rig, then the CPU and graphics may matter more. But you're better off saving $90 to get an i3-2100 and spending that money on graphics if your purpose is gaming. I think I gave you this link before, but you'll see an i3-2100 does great in gaming: http://www.anandtech.com/show/4083/the-sandy-bridge-rev...
Including graphics, what is your total budget? $650? Since your budget is tight, I can guarantee you won't find anything better than the Antec 300 Illusion for $45 w/ free shipping. I recommend you buy that before the deal ends or it goes OOS. Do you have any other parts you can reuse like a case, DVD, or HDD? Make sure your new mobo has connections for reused optical or disk drives.
CPU: $180 i5-2500K
Mobo: $60 Gigabyte GA-Z68MA-D2H-B3 $99.99 AR @ Microcenter w/ $40 off
RAM: $45 8GB (2x4GB) 1600CL9 on sale sometime
HDD: $50 Samsung Spinpoint F3 1TB (wait for sale)
Case: $45 Antec 300 Illusion
PSU: $70 OCZ Fatal1ty 750W
TOTAL: $450 (includes $50 of rebates)
Now spend $200 on graphics and get a GTX 560Ti (includes rebates).
My dad's getting my old computer and 500 dollars is my top limit. I'm planning to wait until Black Friday or Christmas when they have a huge deal on a graphics card and buy it since my dad buys the occassional upgrade.
Oh? If you're willing to wait until Black Friday...well, I hope Bulldozer's out by then. If so, you should see some better prices on graphics either way. So is it $500 for the parts I listed and you already own a 6870? If so, then you could go for the cheaper mobos like the one you linked that only does Crossfire. On the other hand, you'll be able to upgrade to NVidia's Kepler in a year from your single 6870 and might decide to go SLI in 2013--which your i5-2500K will still be able to handle.
Hey, come Black Friday, you might get a 6970 for $200. Last year when GTX 470's were $300, I got mine for $200 on 11/22. Today, Newegg has a 6950 1GB for $190.
Anyways, you'll be best served to put a new build together with the prices available when you're ready to buy in a month or so. Feel free to PM me if you start a thread then. If you want the best combo deals, you should also PM Why_Me and ask him to check out your thread. He REALLY knows Newegg combos.