How on earth did you get 73fps?
Here is my timedemo with the same settings I guess.
Date / Time: 10/2/2003 7:48:04 PM (7729593ms)
2900MHz, 1024MB, 128M ATI Radeon 9800 PRO (DeviceID=0x4e48) Driver=6.14.10.6378 Shader=1.4
C:\Games\Halo\halo.exe -vidmode 800,600,100 -timedemo (Version=1.0.1.580)
Frames=4700
Total Time=119.03s
Average frame rate=39.49fps
Below 5fps= 18% (time) 0% (frames) (22.081s spent in 15 frames)
Below 10fps= 18% (time) 0% (frames)
Below 15fps= 19% (time) 0% (frames)
Below 20fps= 19% (time) 0% (frames)
Below 25fps= 20% (time) 0% (frames)
Below 30fps= 22% (time) 2% (frames)
Below 40fps= 36% (time) 14% (frames)
Below 50fps= 62% (time) 44% (frames)
Below 60fps= 92% (time) 86% (frames)
Memory used Max=159MB, Min=130MB, Ave=148MB