Most modern monitors have the option of VGA or DVI inputs, some fancy ones may even have S-video and RCA component video.
You can use the AV outputs to connect to a TV/VCR or monitor that has S-video or RCA video inputs on it.
With the built-in Mac video port you will need an adapter to a VGA monitor, you can still find these adapters on-line. At one time Viewsonic monitors used to come with them free. They have little switches on them, make sure you set them correctly for the size monitor you are using. The built-in video card has support for probably up to 20-inch 1600x1200 monitors.
http://support.apple.com/specs/powermac/PowerMacintosh_G3MiniTower.html#top
Another option, if you want to to go larger and use the higher quality DVI monitors, or add additional monitors, is to add a PCI video card like the ATI Radeon 9200. This adds additional VGA/DVI/S-video output ports that can be used instead of or in conjuction with the built-in video output ports. The new video card would improve DVD playback and add support for higher resolution monitors.
(If you use the AV for video capture in OS 9, then you must have a monitor connected to the built-in video card, and the capture preview window must be on that display.)