Right Bob, you can only connect a 3.4 client to a 4.5 master (and nothing higher).
Kinda depends on the OS of the client. If it's NT4, it could be a severe pain to upgrade (requires an IE version and SP6 I think) to 4.5 or 5.x.
If your client is 2000 or 2003, then there's probably no reason to not upgrade.
Hehe, was stuck with some 3.4 clients at my last job; they stopped me from upgrading my master from 4.5 -> 5.1