If you're actually gaming across both monitors at 3840x1200 resolution, then 3 GB will probably be enough for most games. But a bigger issue is that the 280X's GPU is not going to be fast enough for that in the first place, unless maybe you added a second in Crossfire. 1440p is pushing it for the 280X, and this is much larger than that.
If you're only going to be gaming in a single 1920x1200 monitor (and using the other for desktop activities) then the VRAM isn't an issue. That's still a single monitor situation, as far as your hardware is concerned.