the 7970 supports 4 monitors. and you can game across all 4 if you wish.
If it's for gaming across multiple display's id reccomend the ATI as Eyefinity is currently much better than Surround
Crossfire 7xxx scales better than SLI 6xx currently so thats another factor to consider.
the effective limit is somwhere between 1 and 2gb of VRAM for 1080P gaming. after that point it makes no difference unless you go to higher resolutions.
Interesting. I was planning on doing crossfire/sli at some point so this may be an issue. Is this problem inherent or will nvidia likely close the gap soon?