That's a choice that you'll have to make for yourself. (of course!) But I think any Doom fan would consider running Doom on an XBox sacrilege.

I am unsure of what the quality settings on the XBOX would be. Anyone remember how much memory the XBOX has?
Doom 3
ULTRA Quality=512mb video card
HIGH= 256mb
Medium= 128
LOW=64mb
Also consider that Deus Ex, Thief 3 on XBOX ran very slowly (no doubt due to the ports being optimized more for the PC). Also I'm wondering if Doom 3 on XBOX will have AA turned on? If so then the image quality would have to be reduced even more so to provide acceptable framerates.
Of course the only way to know for sure is to wait for it to come out and see for yourself.
Consider: The three biggest engines for the next 3-4 years will be the Unreal engine (Which won't have another major revision for another 2+ years, The Doom 3 engine (which probably won't have another revision for 5+ years) and the Half-Life 2 engine for probably won't have another revision for 5+ years).
These engines should run just fine on 2+ Ghz processors and X800, 6800 video cards for the next couple of years.
The only problem with buying an X800, 6800 nowadays is AGP vs PCI. There is no difference in speed but the newer mobos will not have AGP support. (VIA may have an AGP/PCI-E mobo). I believe this won't be a problem since as I stated above 2+ ghz processors and X800/6800 video cards should be good.