I used an MDD G4 as my primary, and sometimes only, computer for 7-8 years. It is definitely loud, but I got used to it. Mine is one of the OS 9-bootable ones Apple released together with the first G5 in summer 2003, so it's a bit of an oddball... it boots OS 9 and has a 167MHz system bus, but has no FW800 and a dual 1.42GHz CPU implanted from a January 2003 FW800 model (I got it new as a single 1.25 and kept it up with modernity the best I could). It got me through high school and college, and it still retains a place of honor under my desk even though I don't really use it anymore.
For OS 9 gaming, though, I'd probably go for one of the earlier G4s. By the time you get to the MDDs, everything was really designed for OS X, so if you want OS 9 gaming, any graphite G4 with a good (for the time) video card and a fast single CPU should do. Are the games you plan to play Mac OS-specific, though? All of my Mac OS 9 games are ports of PC games, and if that's the case for you, too, you might be better off just grabbing/building a new PC and the original versions of the games you want to play. It would be faster, more compact, and more energy-efficient.
If you want a general-purpose computer, I second the suggestion of a Mac Mini.