I run my i7 iMac in 64-bit mode. It might be slightly faster. It's important to realize that the only difference is in the kernel. The rest of the OS and virtually all applications shipped with Snow Leopard are already 64-bit.
Any kernel extensions you have (kext) have to match the kernel. Applications don't care. When I've compiled my 32-bit applications as 64-bit they run slightly faster (even with a 32-bit kernel).
Lion only has a 64-bit kernel.