Switching to 64bit OS might be cool but it is not very easy. Quite a few hardware related software do not work in 64bit windows. I have had to give up my fingerprint reader (from Microsoft) and the wireless telephonehandset that I used with skype (from Linksys).
When running in 64bit mode all application require a lot more of memory and I am starting to feel that 4GB will not be enough. 6GB seem to do it.
An early problem I had was that my computer didn’t see all 4GB of installed memory but that was because I had to set my motherboard to remap memory in the BIOS settings.