If i go Windows 7, i would consider getting 4GB ram
, as for now im still using the good ol WinXP SP3 32bit. 2GB seems fine for me now, i don't have lot's of apps in the background, so the system is only using about 300-400MB of ram, The only large memory hoggers i had so far were Crysis and GTA4 (consumes around 1.4GB+ i think) Fallout 3 only uses around 900-1GB, even with lots of mods.
And yeah, i did placed some 800Mhz DDR2 modules before, problem is it has issues with instabilities with it and wouldn't boot on my old board. Researched online and and found out that some folks are experiencing the same and only some select DDR2 800 works, so i defaulted to DDR2 667 (perhaps some bios flashing and modifying would do make it work...)