Is it possible to play Halo: Combat Evolved with System Link/LAN between an Xbox and an Xbox 360?
Yes. You can play any backwards compatible games (which includes Halo 1) via System Link between an original Xbox and an Xbox 360. You can't just connect the two Ethernet ports together, however as the original Xbox requires a crossover cable. You can however, plug them both into the same router. You will need a disc per console, as with any System link play.