Quite a bit, MSAA is essentially sampling certain items on the screen at higher resolutions/multiple times then compressing them back down to get rid of jaggies. It's pretty VRAM intensive.
Who said these were 980s? Titans and Titan Blacks have 6gb of VRAM.
PC Gamer article stated that the setup was 2 x 980s.
http://www.pcgamer.com/why-gta-5-pc-is-the-definitive-version-of-rockstars-epic/
IGN's article didn't mention anything. That's the info we have right now. The Titans could be an option though, you are right
