How many triangles should a model have total? To be more specific, does anyone know how many triangles/faces the characters in halo (masterchief or AI Characters), Doom 3 or any recent video games have?

Its around 2000 to 20000 vertices.

3000-5000 tris

depends on the engine you're making them for.
some engines will handle a wide range of LODs, and the highest detail LOD might have 20000 triangles, while the lowest detail LOD might have a few hundred triangles only...
it also depends on other of the engine's technologies. for example doom3 can't afford very high-res meshes because of the shadow volume extrusion cost.
and it also depends of course, on your target hardware range...

ingame ...2000 is a safe figure for one of the main characters... though its getting bigger daily

the weps in our game are 500 ... characters 2000-2500 ...npc's are lower...

going high poly and then optimising afterwards seems to be a trend ...what with the whole "normal map/shader" thing going on... (doom3)

It all depends on your target hardware as well as your game style. If you're going to be doing a first person shooter with emphasis on cutscenes, you should probably push the tri count. On the other hand, you could be working on an RTS where super-detailed characters would be a waste of time and processing power. Similarly, Doom3 monsters don't have huge poly counts because it's made up for by normal mapping. Quake I didn't have huge poly counts because it was made to run on an older computer.

Quote: Doom3 monsters don't have huge poly counts because it's made up for by normal mapping.

doom3 models aren't lowpoly because they are normal mapped... in fact doom3 would look much much better with highpoly characters (and a variable specular exponent, but that's irrelevant here..) they are low poly because of the engine, and more precisely because of shadow volumes extrusion. that would be far too slow with high poly characters.
that's another aspect that influences your polycount... it depends on the engine you're using...

...tomato...toe-mat-o...