> whats required to host a forum like this
What the others said. For modern PHP/MySQL you want something more than 486-33, but a P-133 or VIA-800 is plenty. And you can't find a hard drive too small... reliability is the issue, not size.
Traffic: HeadWize used to go down the 29th of the month because he did reach his traffic cap, but that was a couple years back and I think most hosts give you a lot more bandwidth now. I don't recall what his cap was (I may not have known). That was both Forum and an extensive Library, I dunno what the traffic ratio was.
For 0-20 users at the same time, home DSL would be fine, even with the cheezy uplink throttle such accounts have. However you don't get a Fixed IP, which means a little scramble every time your ISP breaks your line to force a new IP on you. This and the uplink throttle are designed to make you buy a Commercial DSL account. In fact Verizon's latest ToS bans all servers on home DSL.
The phpBB website has links to hosts who can get you professionally set-up on a managed server for some modest monthly fee. If you have to ask, this may be the safest bet.
> a Mac G4 400.. Surprised that it's not dead yet as it's in a bad (hot) enviroment
We had a Lab full of G4 400s go through a summer with no A/C under a thin tin roof under a blazing sun. The building got so bad, I just didn't want to go in and shut them down. That was 4 years ago, all machines still run fine, though only two still in active service.