Yeah, ditto to what akira said about the performance of alpha versions, and the doom 3 alpha is quite old by now.
However, I remember when I saw the doom 3 alpha(wasn't there kind of an unofficial ban on talk abut the doom 3 alpha?) on a geforce 3, and I believe a 1.4 gig athlon, it ran like crap, like 10-15 fps. On a 9700 pro with a 2.1 ghz tbred it runs faster than 30 fps most of the time, sometimes 60 fps. In other words, it has performance about on par with halo! ok, not that bad...well, actually at 640x480 in halo I would probably never see a frame under 60, and on a (overclocked to xgpu speeds with default ram)geforce 3 my avg fps in the timedemo at 640x480 is around 40 fps a 1.8 gig tbred.
Hmm, I still wonder, is the xgpu really faster than a geforce 3? I mean, its specs are closest to a geforce 4 ti 4200, but it is even more bandwidth limited than a ti 200, and from my testing it seems that a 230/460 geforce 3 gets exactly the same scores as a 210 or a 250, and only a slight improvement over a 200 mhz. Plus, from the benchmarks I've seen, the doubled shaders of the geforce 4 at best only make like a 20% difference in performance compared to a geforce 3 at the same speed, and at higher resolutions the difference goes down to basically nothing. Just doesn't seem like the xgpu would have the bandwidth to perform even as well as a ti200. Can't really test evenly though, as the pc version of halo runs with a lot less effects(but I can go up to 1024x768 and still keep an average of around 30 fps, or turn on fixed function and go up to any res and stay over that :? ), and I'm not sure what kind of settings NFS:U runs with on the xbox. At high detail I'm stuck at 30 fps at 640x480, but at medium detail I get 60 fps most of the time(sometimes drops slightly), or I can manually set the settings somewhere in between medium and high and get 60 fps. Guess I could try Star Wars:KotOR or Deus Ex:IW demo and see how they perform, but I still don't know what kind of settings they run on xbox. If xbox runs NFS:U at high settings, then I guess the performance is very indicative of what the doubled shaders can do.