Your save games can be used w/o a problem, you should be fine with 64 bit 2 gb ram, but it will definitely perform better with more ram, especially for gaming.
Nowadays 64 bit os is common, and compatible with almost all softwares, i havent got a single problem with 64 bit os- So no worries here-
But what heavy games are running on 2GB of Ram? To run heavy games you need 4GB of RAM. That is, using win7, 1GB for the Windows, 3 GB for the game. If you have 2GB, only ~1GB is left. Forcing Cpu to page file and causes misc.stuttering in-game. Ex. when grenade explosion you will experience stutters.
I'd say there won't be a difference, but for a little less performance gained. It's barely any difference between the 32 and 64 bit platforms except for the amount of RAM usable(which in your case with a 2gb ram installed would make no noticable positive effect). Stick along with a 32bit version until you increase to alteast 4gb RAM.