There are only two versions of OS X, consumer (you and me) and Server.
Both are 64bit, if your machine can handle it, and both can use all the memory that you can fit in it. Up to the spec limit, of course.
I'm thinking about getting a Macbook Pro and one of the major selling points for me is that you can get 4 GB of RAM for relatively cheap. What I wanted to know is: Can the Leopard OS use all that RAM or is it like Windows where you can only use about 3 GB of that with the normal 32 bit OS? I bought a Dell XPS 720 last summer and was really pissed off when I found out that I had to install a new 64 bit OS to utilize all of its RAM. Do Macs work this way too?