Hi Nick - we have made the change in our office recently from 32 bit to 64 bit, but the process is being staged, so at the moment we have users on both with no problems. Bear in mind this is with the commercial version, but I doubt there'd be any differences.
We are still using artlantis 32bit though - Artlantis 64 bit is only available as an Alpha version, so I can't comment on this one.
cheers...