I installed UT GOTY just for kicks, and at 800x600 in a window in XP in Parallels, it's very fast using just the software renderer.
However, I can't control anything. Whenever it goes to "full screen" mode inside parallels, the mouse becomes uncontrollable. I just spin in circles. This is true of both the Bluetooth Mighty Mouse as well as a USB Logitech optical mouse. Why? Works fine in XP, or in UT when the game menu is in windowed mode.
I'm impressed it works so well, because on my MacBook it would crash in BootCamp 1.0.
Now to install UT 2004...