I found an Unreal Tournament port of Marathon a little while back. Quite possibly the best-looking, and best-playing, Marathon experience I've ever had. They completely redid EVERYTHING, from models to physics to maps, while keeping it as true to the original as possible. Unfortunately, they never released anything more than the M1 demo (that I could find, at least) and I haven't seen anymore of it.
I can't remember the name of the group or the mod, but I can look it up... anybody interested?
Oh, and Niallerc: I played Halo at MWSF last year on a G5 and it played great. Perhaps there is some sort of optimization you need to do?