I have 16GB and I don't think I've ever seen it used fully, except when I work on high resolution RAWs. But even then it's probably only caching excessive amounts of data.
In real life you're not likely to see a difference.
And, 7970 Vapor-X GHZ ed is a solid choice.
I use 7950 Vapor-X overclocked to more or less 7970 GHz clocks and game at 2560x1440. I'd buy it again, and by the time I'll need to put a second one in, I hope AMD will be done with fixing Crossfire .