Unfortunately I don't have time to join in the fun, so I'll leave some tips I know to help those who are participating.
Many years ago I had a small site about mods and hacks to run lowspec games, it was mainly focused on running games on the worst vgas like intel and sis/via.
I won't mention the most common tweaks, as they have already been mentioned in other posts.
Open the game's textures, and reduce them to a ridiculous size, like 16x16, 8x8, or 4x4, you'll get something like a nintendo 64.
Disable all effects that use alpha channel.
If I'm not mistaken, there is a way to disable animations in quake.
Use Windows XP, and disable EVERYTHING that is not needed for the game to work. It is possible to leave the ram consumption below 32mb with tweaks easily found out there.
I didn't see (or didn't pay attention to) rules about macs. Macs are much more effective at processing math, and power per clock. A g4 800mhz, roughly equivalent to a Pentium 3 1400 or P4 1600mhz. The only problem with macs, are the poor graphics drivers. But quake is from iD, and iD used to polish and take every last drop of performance out of macs, so that won't be a problem.
I don't know if this would be cheating, but there were some multi gpu video cards in the past, they were designed to use multiple monitors, but there were modified drivers that made them work as SLI (with a cost of this management falling on the processor).
Have your machine on a table, without the case, with a ventilator.