I find changing it to Direct X 7 mode in the video options makes it run far better and the difference is minimal.
On my desktop at home I actually took some screen shots to compare the differences and the only thing that stood out to me was that the walls had a much more 3D texture with DX9 than DX7. Still, I would much prefere the faster framerate and a higher resolution than nicer looking walls.
The performance difference I experienced was about 20-25 fps (frames per second) with DX9, while at DX7, it frequently was maxed out at 90. That was with a Geforce 6600GT.
If you want to see what frame rate you are getting, press the ` key to open the console while in a game (that is the key to the left of '1') then type 'cgdraw_fps 1'.