I get motion sickness too, in very nearly all shooter games. For me, no configuration of proper lighting or Ginger Pills has helped. I first noticed it when playing Halo for the first time (so I wasn't always like this). I think it has a lot to do with the in-game surroundings; the level design in Halo 1 repeated itself a lot, and I had a hard time figuring which way to go. Anyway, games like Portal are out of the question for me. Team Fortress 2 gives me no trouble after I get familiar with a new map, but if I'm playing a shooter where I don't already know where I'm going and can't check the level map every 5 seconds... Well, let's just say I better have a bucket nearby.
I've found my body can build up a sort of tolerance for it; if I play often, it takes longer and longer each time for the nausea to hit me, so in the end I'll no longer start feeling ill at all. If I switch games or have a week's break from a game, though, I'll have to start from scratch.