I'm quite aware of the 1.83 GHz model, I was just being nitpicky. 😉
If you're playing Halo PC you don't need the UB, because you can't run that under Mac OS X anyways. Now, if you want to run the Mac edition of Halo, you don't need it either if you don't mind sacrificing even more performance. But for optimum play yes, you do need it.
You change the details by going to preferences – it's very simple.