Jim is correct about restricted memory use ( I was adding totals and taking into account apps that can go over the 3g mark) and although a large address application can grab just over 3 gigs there are not many out there able to do that and remain stable. FS9 wont. Having more than 2gig for FS9 is really not necessary at all. FSX on the other hand will take advantage of over 2g, especially with SP2/Accel installed because it has the 2+G marker in the exe file.
Even if you set up the system correctly and use the right tweaks FS9 wont use over 2G