To run a 64-bit os with 32-bit userland is easy, and also the current OS X way (might change in Snow Leopard for all I know).
Just compile a 64-bit kernel and modules, boot TC
The advantages on the other hand - more registers, ability to use more ram better. The apps themselves would still be limited to 4gb ram as they are 32-bit apps, but the kernel would use all ram efficiently (unlike with PAE which is slow).
I have so far to see anyone running TC with over 4gb of ram, and the couple extra registers hardly make a difference.