AMD and Nvidia each have their advantages. If you're planning on playing games that support PhysX, or happen to Fold a lot, or want to make use of NVidia's 3D Vision then NVidia has the advantage. If you want multiple monitor setups (Eyefinity), excellent bang for buck cards, then AMD has the edge.
You can't unlock the Flex card, that's why I was originally going to get Sapphires normal 6950 card (the one everyone's buying) and unlock it to a 6970.
I don't think any of you caught my other question up there but should I get a CPU heat/sink fan if the case I'm getting has a ton of air flow (two 250mm fans and a few 120mm fans)?
Finding a reference design card that can unlock these days can be difficult. I have one myself and have done the 6950 BIOS flash to unlock the shaders then overclocked it. The Radeon 6970 BIOS flash method is a risk to me due to different memory timings. The difference is about 5-10% only though. A card like the MSI Radeon 6950 Twin Frozr can actually be overclocked to achieve Radeon 6970 performance these days:
http://www.newegg.com/Product/Product.aspx?Item=N82E16814127588&cm_re=radeon_6950-_-14-127-588-_-Product
The 2GB version is very useful at high resolutions...at the very least 1080p resolution (1920x1080). For lower res, you can settle for the 1GB version although it will be fine for 1080p as well.
AS mentioned, CPU HSF if you plan to overclock...otherwise stock fan will be fine.
Oh alright. And another thing (sorry for all the questions

) but I'm having trouble finding the right motherboard. I pretty much have it narrowed down to http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&N=100007625%20600007943%204802%20600007977&IsNodeId=1&name=2600MHz%20Hyper%20Transport%20%285200%20MT%2fs%29, however I cant really tell the difference between most of them.
AMD Phenom chip? Should have went for Intel's Sandybridge if you're builkding a PC at this moment. The 2500K and 2600K destroy everything on AMD's front atm...at least until Bulldozer releases.
Differences between those boards are the features. Number of PCI-E slots, SATA ports, USB ports, VRM phases (matters for OCing), PCI slots, crossfire support, etc. Don't bother with the overpriced Asus Maximus board. The 880G and 890 series chipsets are decent enough. 870 for budget and basic boards. Comes down to the features you want. Again though, you're looking at old technology here....AMD is moving on. Intel is where it's at right now.