I really appreciate everyone's feedback on this. Just to let everyone know, I stopped at Radio Shack after work today and got both the XPR113 (Xenon) and the HPR50 (Halogen).
After trying both, I have to agree that the XPR113 is by far the best choice for my ArcWhite. Brightness, whiteness, and even overall shape of the light is definately better than the factory bulb (which really wasn't too shabby to start with).
I'm getting a nice, uniform round beam that that spreads out nicely from the hot center.
The HPR50 may have been slightly whiter, but I really couldn't tell if it was brighter, or not. The beam was very rough and uneven (probably not good focus) with odd looking "cold" streaks through it. When I get time, I'll try it on some of my other 6V lights.
In all, I'm very pleased with the results. Thank you all!