I've done a clean re-install of OS X Snow Leopard, so there's nothing on my system at all.
I've installed Xcode and I'm ready to install Rails.
However, my system is a MacBook (MacBook2,1) and apparently it doesn't support 64-bit by default. Apple is going to add this at a later date.
How would I go about installing Rails? Do I go about it the normal way?
How about MySQL, do I need the 64-bit binary?
You can certainly install the 64-bit packages. They will be able to access the memory space and every advantage of running with 64bits. The only thing that won't boot in 64bit is the Kernel, but it's not something to be worrying about for the moment.
Check out the Hivelogic's guide for installing MySQL, Rails and Ruby on Snow Leopard: