You could afford a 6990 for the price of those two 6970s. Also, you may be getting 50% more memory bandwidth with triple channel, but it's not like your processor will make up for it and give you par performance now that Sandy Bridge is around. Two 6990s barely saturate 16 PCIe lanes so it's not like you'll need that many lanes with two 6970s. The i7 950, and even the 960, would be the bottleneck.
If you're still set on TRIPPAHL CHANNAHL memory, just stick with a single 6970 and consider this a decent mid range machine to keep you going till whatever comes out to replace LGA 2011 and the Sandy Bridge Enthusiast chips due in 5 or so months. You say you've done your research and looking for concerns about the parts. Keep in mind that only the enthusiast level of hardware can make full use of the chipset (i7 990x, dual 6990s, RAID, etc.). You'd get much better performance per dollar out of Sandy Bridge.
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