I can tell you about Quake 1. Quake 2 I have sitting on a file here somewhere and haven't gotten around to trying it, and I have no clue about Quake 3.
Correct me if I'm wrong on any of these things.
Quake 1 is made up of a game engine and maps. All these maps are in a folder in the Quake directory called "ID1", with other folders for other expansion packs and so on.
By downloading Quake for OS X from VersionTracker, all I needed to do was import the ID1 directory. I happened to have the PC version, so I wrote the ID1 file to a CD from a PC, then loaded it into my iBook 500. You could do the same to import from a Mac version of Quake.
The ID1 folder is somewhere in the neighbourhood of 50 megs. I think the shareware version was about 12 megs, and I don't know if the port to OS X will or won't run the shareware version. Either way, Quake for PC is cheap and easy to buy, being a dated but very popular game.
The download for OS X port of Quake is here:
http://www.versiontracker.com/moreinfo.fcgi?id=11413&db=mac
The setup is relatively simple and the port works beautifully. Basically, if your machine runs OS X well, it will have no real hassle running Quake.
Anyhow, I'm sure the
real gamers here will fill you in a bit more...