First you need to modify your console by chipping it or softmodding it. If you're good at soldering, chip it. But if you're better with software, softmod it.
Go to www.xbox-scene.com for EVERYTHING you need to know. They'll show you how to mod it. As for obtaining Quake 2X, it was probably made with the XDK, which is illegal for most homebrewers to use, so you need to get it in "the usual places". There are tutuorials on how to use "the usual places" on www.xbox-scene.com as well.