for heavy gaming you should have more then 2gb. If it was just some light gaming i would say it is okay, but with games like gta4 and crysis. You should be working on getting and extra 2gb's.
mmm apparently games like Microsoft flight simulator are big memory hog so for games like that you would need more. But yeah some games need more of hardware than other. GTA IV is a big processor hog, MFS needs a lot of RAM, crysis needs everything ^^
RAM is relatively cheap now so manufacturers throw in a lot more and charge you $100 extra.4Gb seems to be the norm today in most systems.but since the i7 release alot more systems,even pre-built ones are comming with 6-8gb,
with the price of DDR2 I would get 2x2GB sticks.
What most say is
Vista = 4GB
7 = 6GB
As to why I have no clue but if I had to guess most using 7 would have a I5 / I7 (which tri SLI = 6)