Are there any useful, cheap mods for the 333Mhz iMac G3 compatible with OS 9?

Bump up the RAM, hard drive and optical drive. Beyond that, you leave the realm of cheap. There were processor upgrades for the tray-loader iMacs, but if you can find one at all, the price will be sky-high.

I wouldn't invest money in an iMac of that vintage unless you're smitten with the form factor. The analog boards have a tendency to expire, and faster Macs are cheap and plentiful on the used market.

A cheap G4, a jigsaw and some rubbing alcohol will let you clear case mod.

There's also the ol' CPU speed bump; I've got a couple of 333 CPUs running at 400 without issues. Cheap it is, just needing a _grounded_ soldering pencil (or two, 2nd makes it much easier to lift tiny resistors.)

400MHz is very likely, but you could try for an even higher speed, those G3s are pretty tolerant. My 400s can _almost_ smoothly play video files (avi, mp4, etc.) Playback still stutters sometimes, but is noticeably better than at 333. Mind you, this under X, most of those modernish codecs just don't exist for any classic MOS.

Original "tune-up" site has been gone a couple of years now, but lives on at the wayback machine.

dan k

ps: your OP asks about compatibility with 9: yes this bump works fine with any version OS