32 bit software runs perfectly fine on a 64 bit system,
Only differences are that 64 bit supports more ram according to MS its a licensing issue or something or a hardware issue but server 08 bin compatible with vista but doesn't have any ram caps.
MS aside the difference between 32/64 is in a 64 bit system you need a 64 bit capable processor generally this isn't an issue nowadays, but there is hardly if any true 64 bit software available to the consumer. But when booting a 64 bit os you need to boot a whole other HAl along with your base os to run your 64bit apps.
Generally I try and run 32 bit when ever I can because the performance gain of not having services run side by side that I don't, or can't even utilize pays out.
I have noticed that with certain .deb packages that are compiled for i386 will not run on a system that has a 64 bit Debian based operating system installed namely flash plug-ins and IBM lotus symphony.
Generally with Windows this isn't the case though if you have a 32 bit package it runs from your c:\program files\x86
your 64 bit packages get installed to c:\program files\x64
hope this helps If your computer comes with x64 os just leave it be if you're not shure and don't rock the boat