Well, I use 64bit OS on my PCs that have more than 3 GB of RAM, bacuse using it with less would be slower than the 32 bit version.
I still use 32 bit version on an old Pentium 4 due to some 16 bit programs. (compatibility reasons).
For seeing differences, by the way, you must use also the programs at 64 bit, for example when downloading Firefox, Chrome or VLC you must go to advanced options ad choose the 64bit installer, otherwise your PC will run in 32 bit emulation.
For Chrome, I noticed about a 40% better rendering performance with the 64bit, using the full power of my 8 cores!
Cheers!