Both Nvidia and AMD have good offerings at the moment. I believe that HardOCP and Tom's Hardware are both recommending the Geforce 670 or 680 as the best bang for your buck currently. But, the Radeon HD 7970 isn't a bad choice now that the drivers have matured a bit.
You should check to see if your monitors are going to be compatible with both before making a decision. Eyefinity will require that at least one monitor has a DisplayPort connection or you will need to use an active DisplayPort adapter. My Eyefinity setup has a native DisplayPort on the center monitor, so I can't comment too much on adapters. But, I have seen a number of posts from folks having problems with unreliable or failing adapters. On the Nvidia Surround side of things you'll need to make sure all three of your monitors have the same sync polarity. Unfortunately, this can be tricky to verify and you can even have multiple monitors of the same model with different sync polarity. It may not be mentioned in your monitor's manual and you should use a program like
MonInfo/Monitor Asset Manager to double-check before making a purchase.
Extra RAM will help with higher resolutions and anti-aliasing, but you're correct that Crossfire and SLI do not stack RAM since each card has to have the same data in RAM. So, if you plan to go higher than 5760x1080 or want to be able to use anti-aliasing, I'd suggest going with a 3 GB or higher card. Crossfire and SLI will help out a lot in games that are properly optimized for them. But, there are problems. Lots of people are sensitive to microstuttering caused by running in SLI or Crossfire. On most motherboards you're going to be placing two cards in Crossfire or SLI so close together that airflow will be severely limited on the top card, causing higher temperatures. I've tried single and dual 5870s and I prefer the extra performance with two, despite the drawbacks. But, I'll probably start with a single card next time I upgrade. You can always add a second card later, if you need more performance.