The Retail version, you can install it on a pc, then if you want, move it to another pc later if you get a new one.
OEM is locked to the first hardware it is installed on. Cannot be moved to another pc.
(depending on the situation. ie: hardware failure or such could persuade Microsoft to let you move it, but not without some work)
32bit Operating Systems are limited to 4Gigs of RAM, and some of that gets reserved for OS resources, reducing the actual RAM used by software and applications to around 3Gigs or so.
This limitation is not present in 64bit Operating Systems, and the 64bit architecture allows some programs to run faster.
In a nutshell.