I need to get a new video card for Starcraft 2. I mean, that's not the only thing I'm going to do with my computer, but just wondering if any of these are OK and what's the difference...for games and other things.
You may as well save a bit more and get a DX 11 card as hell_storm2004 says. But in answer to your question, 1GB is really only advantageous against 512MB at larger resolutions as a larger frame buffer size (in MB) per frame is required compared to lower resolutions.
Oh, btw, what happens when you do something like 2 x 9800 GT in SLI mode? What would that be quivalent to? Is that actually good compared to a single video card of the same total price? Or is that just a marketing gimmick? Would you be able to like run each monitor off one card in like a multi-client scenario?