If you want to play it safe, in iTunes there's a button "consolidate library" which puts all your iTunes songs/movies/etc. into one folder - which can then be transferred to a external hard drive.
I have a 64bit installation on my current computer and I can't tell much difference performance wise. I'd install 64bit just to future proof yourself, as eventually everything will be 64 bit.
As kemical said, I would do 64 bit just to future proof yourself. I have a 64 bit installation on my current computer and I really can't tell much difference anyway.