Even at 4 gigs of ram it really doesn't give any benefit because your 1GB of video memory (I'm pretty sure) reserves a chunk of your regular system memory even though windows doesn't show it as "in use".
Also, as stated before, pointer sizes double, increasing the memory used by windows and your game for the same functionality, the extra 500MB that windows will address is still not very beneficial. 4GB isn't a bad idea to add, but you don't really need to switch to 64 bit unless you go higher than 4gb.
If your system is just running really slow, the ram at 4GB and a new GPU are where I'd start. A 64 bit OS really won't help until you have 6GB or more. (Arguably, even having 8Gb you may not see much of a benefit vs 4gb, 64bit os or not, because in order to improve things, you have to need that much ram).
edit: If you are a student with access to FREE OS's, you could make a case for switching to 64bit windows IF you have 4GB of RAM, but your benefit will be negligible (or non existant). Other than legacy software (which you CAN emulate for if you really want to via dosbox or virtual machine of winxp i think), there really aren't any drawbacks to 64bit win7 (Antivirus can be a sticking point too, but microsoft security essentials is free and nice and has 64bit support). I wouldn't pay any real $$ were I you to make the switch though.