Not once has someone mentioned southbridges. I might tear up here.
If your motherboard supports x16 by 16x SLi or Crossfire operation, then you're in the golden state, my friend. Most motherboards split the PCIe lanes between the two cards and both cards operate at 8x.
Anyways, are you talking about, for instance, A) [1x HD 3870 1GB] vs. [2x HD 3870 512MB] or are you talking about B) [1x HD 3870X2 1GB] vs. [2x HD 3870 512MB]? I'm just using the 3870 cards as an example (there are other dual GPU cards out there for ATi). If you're talking about option A, then you'd be better off with [2x HD 3870 512MB]. If you're talking about option B, whatever floats your boat; I'm too lazy to research which is better.
Honestly, though, RAM is definitely not what you should be looking at when it comes to video cards. As long as it's GDDR3 or GDDR4 and 256MB+, you're fine. 256MB is plenty to run any game at 1024x768.
More important factors to look at while shopping for a video cards are: GPU (What model is it? How well does that model do compared to others?), Memory Interface/Bus (128-bit cards are dating quickly, and aren't as powerful as their 256-bit counterparts), and Heat (depending on if you care or not).
If you want some help on video card shopping I'd be glad to point you in the right direction (I'm sure that other members would love to help out, too), just post back with your computer specs and ATi or nVidia preference, as well as price range.