I think if you're ever going to want more than 3Gb of system memory, you'll need to have a 64-Bit version of windows. Not sure about Windows 7, but I think 32-Bit Vista only supports up to 3Gb. (Although I've also read that 32-bit can also support up to 4Gb, not sure which is accurate)
It's not a matter of XP, Vista, Windows 7, MacOS, or Linux or whatever. 32-bit is limited by the Laws of Physics and math to 4Gb (2^32 = 4,294,967,296), although you will not see the full 4Gb due to overhead - typically you will see between 2.8 and 3.5Gb, depending on your graphics card and how it "shares" your RAM.
Did I mention Drivers
+ 64-bit software support?
Maybe once or twice, as did I in my first post above, and as SURF alluded to with his 7 year old printer example. But to reiterate, if you want to move to 64-bit, make sure 64-bit drivers for all your hardware devices are available first.