What is the difference between 32 and 64 bit?
64 bit allows you to access more than 4gb of RAM and utilises your hardware more efficiently.
Have been told I can only do a 32 bit install.
Depends... via the upgrade path you can only go from 32 bit to 32 bit, etc. If installing from a full retail copy however, you can go 32 or 64 bit. If going the OEM route, please ensure you get/order the bit version you want, as it ie either one or the other... not both.