You won't be playing any modern games on it (even more so than most macs). The integrated graphics they used are pretty good for video, but abysmal for 3D gaming.
Unless you have a keyboard mouse and monitor the mac mini isn't that great of a deal. I personaly would rather get an iMac if I didn't have any extras. Though if you arn't ever going to play games or store alot of movies or music the mini might be better.
Do you know for sure that Doom3 runs ok on a G4 Mac Mini? If so, at what resolutions? My Athlon XP 2500 + a 128mb Radeon 9800 Pro and 512mb ram had some troubles playing Doom3 at a reasonable resolution until I upped the ram to 1gb, then I could play it *ok*
That's pretty neat, then! I guess that an Intel Mac Mini might be an even more capable gaming machine for the casual gamer. It's not gonna satisfy the dudes who want their resolutions huge and all the effects turned way up, but for those people who want the Mac Mini mainly for productive uses I guessing that being able to play the odd game here & there would still be welcome.