Hi Ned
I’ve used Ati cards for my own machines since the 8500, and have always been satisfied with them. I have had a fair amount of experience with nVidia cards as well in various builds for others, and to be honest, there’s not a lot of difference between the two. I’ve always preferred Ati because of their more efficient approach (which typically results in better pricing), but few users would be aware or even care about the architecture of whatever card they may have, as long as it works OK. In the end it boils down to personal preference, and it’s usually easier to decide on a product that’s familiar. Being an AMD fan as well makes the choice somewhat easier for me since they acquired Ati, but you are right about the 990 chipset – my board supports Sli too, so you are no longer forced to buy Intel if you want to go this route.
As far as the 6970 goes, its performance generally falls between the GeForce 570 and 580, and locally it’s priced a fair bit cheaper than the 570, so a no-brainer in my case. My son has a MSI HD5870 Lightning I’ve used for comparison, and it is slightly faster than his in DX9 games (which are most modern games), but does allow higher detail settings in CloD due to the additional memory, which is what I was interested in. It is a fair bit longer than the 5870 (and my previous 5850), which required me to mod my case in order for it to fit, but at least the power requirement isn’t any greater.
In my opinion, you won’t really experience a huge increase in frame rates over your 470 in CloD, although you will probably be able to turn some detail up without suffering any loss (probably not what you wanted to hear if you were intending buying a new card

) I replaced my 5850 because my daughter needed a new card (her 512MB 3870 was getting a bit long in the tooth), rather than wanting more performance. If I had done it for solely for more speed I’d have been disappointed, but that is always the case when replacing a high end card with a newer model.
I hope this has been somewhat helpful.