I got dizzy trying to replay HL1 recently. I think its a combo of the low res textures at a high resolution with a high framerate and the FoV.
I dont know if the Xbox version has it, but check if theres a FoV slider. I know theres one in TF2, and I'd guess its there for Portal and EP2. Low FoV gives me a headache, like playing Halo (has a 75fov). I didnt really have any trouble with Portal, or Prey for that matter with the gravrails.
One thing that is guaranteed to make me sick, though, is keyboard look and to a lesser extent gamepad look. Since it reaches a max speed and is so...uniform. Ugh. I once watched my brother play Half Life with only the keyboard and I could take like 10 seconds of it before I felt sick.