Quake 2 and some other OpenGL games are way too dark on my laptop, and I'm wondering if the 4500mhd is to blame.

Anyone run Anachronox on this card and can tell me how it looks?

Try going into the video options ingame and upping the gamma a bit first. Also I am guessing you are on a lappy, usually on most laptops / notebooks holding FN and I think its F7 and F8 (It is on my HP lappy but different manufacturers may use different keys) adjusts the brightness of the screen. Just look for a little square on the F key with a sun type symbol in it with an arrow up and down.

I actually have my laptop connected to a monitor, and in these cases the monitor brightness setting doesn't help.

See I have a similar issue with my screen. I have a Asus 1920x1080 screen and when I play OpenGL games it sets it really dark even if I up the gamma. I have to go into its menu and change the mode from normal to game or night to get things a little brighter.