I cannot stand anymore "I hate halo" talk. All nonsense. At the very least respect it as a good and innovative shooter, but grow up people.
Processor speed doesn't matter. A G5 and G4 will play it pretty much the same. Graphics card is what really matters, and its possible OEM cards work better but maybe not.
My eMac 1GHz (640mb ram, radeon 7500 32mb) runs it with absolutely no problems, skips, bumps at all on the lowest settings. Only problem I've had is some weird drawing issues where objects became transparent and I could see what was behind them, but only twice. Also for some reason on multiplayer games rocks will occasionally change color for a second.