Well, I was really desperate with OOMs and make all changes I found in the net, 12Gbs was my last atempt, I bough the +6gbs on a easy-return shop just in case, but they did the trick !!! :( My FSX and OS disk are on a pair of 150Gbs WD1500HLFS Velociraptors on P6T
My experience both in Vista Ultimate x64 and Win7 Ultimate x64 is 12gbs got me rid of OOM errors in FSX. I have GEX, UT, FS Global 2010, REX, and My Traffic X installed and perhaps I