cppc, please see this:http://www.tomshardware.com/forum/315300-33-heck-eyefinity
Eyefinity has a lot of weird quirks about it. In summary:
Your assumption with the 6970 is wrong. You will have to have ALL 4 monitors connected to the FIRST video card. You will need a card that has a MINIMUM of 2 Displayport adapters.
The 6970 does indeed have 2 mini displayports. However, mini DP -> DVI adapters, ACTIVE ones (not passive, those won't work for what you need) aren't that expensive. http://www.newegg.com/Product/Product.aspx?Item=N82E16815309008&cm_re=displayport_adapter-_-15-309-008-_-Product
They're $27each. Buy two.
Having two 6970's in crossfire is still good in order to support that HUGE resolution, so don't rule that out, just know that all 4 monitors have to be connected to the first video card. When you crossfire, you're essentially taking two GPU's and making them act as one. So any GPU intensive work should have an increase in performance with crossfired cards.