64-bit operating systems can address systems with 4GB of RAM or more; 32-bit operating systems generally won't 'see' more than 3GB or so on a machine regardless of how much is actually there. I don't know if 32-bit systems place a limit on how much RAM single programs are using but I can't imagine anyone would try to run a single program taking up 2-3GB of RAM with a machine that can only use around 3GB.
As for emulators, unless you've got something else clogging up your memory (a bunch of memory-intense programs running in the background, especially web browsers) there's not going to be a noticeable improvement in most cases.