Have you tried Halo on your Intel Mac (without any updates--just the original PPC version)?
Most PPC software runs just fine on Intel Macs, thanks to the built-in Rosetta PPC emulation.
Games, in my experience, run better (by better, I mean more usably) than other PPC applications. This is because of their dependence on graphics cards, rather than CPUs, which require no emulation.
That said, the Intel update would certainly be worth the $5.