when im playing a game, im mostly listening to music while steam, keyboard and mouse software, antivirus, nvidia and some other software are still running. these program's take around 1.7gb's of my total ram.
so when i run a game running in 32bit, the game is not locked to 2 gigs of overall memory but rather 2 gigs of additional memory. so 1.7gigs of various programs + the 2 gigs of extra memory.. thats why there are games that require 4gigs of ram while still running on a 32bit exe. you could also run the game with 3gigs if you close you're steam, anti virus and other stuff.
so having a 64bit for this game is only good for those who have 4gb's or more memory availble.
ps : a 64bit version will also mean open cities..
... the link that guy posted says basically the same thing