IllegalPaladin on 07 February 2010
Mainly the difference between 32-bit and 64-bit is how much RAM it can see. In 32-bit, you can only see around 3-4gigs of RAM. In 64-bit, you can use a LOT more ram (depending on the OS, ex- Windows 7 home, professional, ect).
This can be a useful chart,
http://en.wikipedia.org/wiki/Windows_7_editions#Comparison_chart
64-bit could also have some compatibility issues with older programs (and also requires drivers to be signed), but one way you can get around that is by dual booting or using Windows 7 pro's XP-mode for the software you want to use (I dual boot and my CPU doesn't support virtualization so I don't know much about XP mode).
Essay Pro