Macs of that age often had "ADC" as a digital video out + display power + USB. It looks kinda like DVI but with rounded ends and more pins. Check the other port on the card more closely.
I could have sworn that all those cards also had either VGA or DVI as well depending on the age. You can get Mac compatible cards as well as sometimes flash Mac firmware to a PC one in some cases. A regular bog standard PC AGP card will not work without modification. Your model should have an AGP 4x slot and four 33MHz 64bit PCI slots, btw. Also three regular PC133 SDRAM slots.
If it is stock it likely has Rage 128 Pro, Radeon, GeForce 2MX... I forget what others. If you are lucky someone might have tossed in a R7500 or GF4Ti or something from a newer model.
These days a G4 like that really is only good for running old software for fun&curiosity though, so it sounds like you have the right expectation.
A 733MHz tower is, IIRC, going to be either a high end "Digital Audio" model(silver side panels, graphite front/top/back), or the low end "Quicksilver 2001" model.(same silver everywhere)
You can run between Mac OS 9.2 and Mac OS 10.4 on that. 10.5 with a little extra work, but I would say to not really bother because it is going to struggle with software from 10.5's day anyway. If you want to run OS X do not bother with 10.0-10.2, they are slow. Stick to 9.2.2 and/or 10.3-10.4.