Is Firefox 5 beta 2 32-bit only? I can't seem to find an x64 download.
You can get the latest Firefox (Nightly) from their server.
The file is called firefox-7.0a1.en-US.win64-x86_64.installer.exe
You can find full Firefox files here
For Firefox 5, Beta 2, I only see one version available from here.
Beta 3 is not available for Windows as of yet.
I do recommend the nightly build if you want to be on the bleeding-edge. You get the latest features and tests before anyone else, and it will automatically update at start if there are changes. Remember though, there are some bugs you may come across using it (since Nightly is their testing stuff), so if you do find bugs, report them to the developers!
Now since you asked the difference between 32 and 64 bit versions, here's a little information from Microsoft:
With a 32bit version of Firefox though, you should not see any issues since it does not access huge amounts of information at a time from the RAM.