OldUnreal (Engine)

I installed the latest OldUnreal version on a (quite old) computer, with the following specs:

Processor:     Pentium M, 1.73 GHz
RAM:            512 MB
Video card:    Geforce 6200 Go
Video drivers: ForceWare 78.01
OS:              Windows XP Home with SP2

As long as the screen is only displaying level geometry, everything is fine. If there are several actors within the geometry, however, it glitches all up. For example: in the NyLeve map, everything is fine until (and including to) the moment I meet the first brute. However, when a second brute shows up, the polygons on the screen start flashing randomly and triangles are rendered where they shouldn't be. The following two screenshots have been taken when there were two brutes on the screen at the same time:
http://oi44.tinypic.com/2vvw0i8.jpg
http://oi44.tinypic.com/2ewfuyh.jpg

I use the Direct3D 9 renderer and these are my settings:
http://oi44.tinypic.com/xc2a79.jpg
What is causing the glitches? What settings should I change?

Try to use OpenGL.
Do you have any graphical glitches in other games?

Try to use OpenGL.

OpenGL solves that problem, but messes up the lighting (many triangles are dark even when they should be lit and the shadows are not applied correctly, see http://oi39.tinypic.com/r9g7sj.jpg )

Do you have any graphical glitches in other games?

No, this is the only one.

EDIT: If I use the Direct3D 8 renderer, the glitches are less noticeable, but they still occur (although only with a higher number of actors on screen). The "solution" of using another renderer altogether perplexes me. Are those glitches an inherent flaw of the Direct3D 9 renderer?

although the game is old, the renderers are not (well not THAT old at least)- means they need probably more recent drivers than the one you have. Updating could fix all the problems at once.
Other than that you probably have only the chance to go back to old Direct3D, which is still in the state it was in 226.

I solved the problem by installing dgVoodoo in the same directory as unreal.exe, then selecting 3DFX Glide from the renderer list.
Now I think it's the video card's fault (maybe because it's a laptop version) because I tried the same version on another computer with a different nVidia video card but the same drivers, and on that computer it worked perfectly.

still possible that the card driver is only messed with the mobile version of the graphics chip, could be worth the try nevertheless.
But in any case, I'm happy it works now for you - for that needs / purposes we still keep the old renderers included and unchanged.

For use Direct3D 9 you ned to have directx9.