I actually read a recent study somewhere that proved that you could run pretty much any game just fine with just 3 GB of ram, I'll see if I can find the study.
Also, as Kashemia said, if you go over 4 GB you'll want 64 bit. 32 bit can only assign memory usage up to 4 GB, so you end up with about 3.5 GB available ram...
So, Kashemia, you should be able to get a 2GB stick of ram for less than $50 and won't even need to worry about upgrading to 64 bit. 4 GB should be able to play the game just fine, especially if that's the only aspect you're worrying about...
Also, as Kashemia said, if you go over 4 GB you'll want 64 bit. 32 bit can only assign memory usage up to 4 GB, so you end up with about 3.5 GB available ram...
So, Kashemia, you should be able to get a 2GB stick of ram for less than $50 and won't even need to worry about upgrading to 64 bit. 4 GB should be able to play the game just fine, especially if that's the only aspect you're worrying about...