The following TR has about the most in-depth discussion I've seen about 32/64-bit aggregates:
The bottom of page 8 has a "benefits of 64-bit" section and there's also good background information on requirements, procedures, etc.
Since 32-bit support doesn't drop off until 8.3 (which is a non-issue for 7mode) you're not "forced" to go to 64-bit. However, if you wanted to perform a copy-free transition of this system over to ONTAP9 or something else along those lines, you'd need to be 64-bit. That said, as JGPSHNTAP points out, 64-bit aggregates are really the way to go, especially if you're anywhere near the aggregate capacity on what you're running and might want to add new disk/etc (and might bump into the 16TB limit).