I went from 1GB to 2GB just last week. Saw (some) improvements in MS Flightsim X and in the IL-2 series of flightsims as well. Don't know about M:TW2, but I'm looking forward to finding out.

I think going from 1GB to 2GB at this point is still a bit of a luxury, whereas going from 512MB to 1GB is a necessity.