I'm also a victim of motion sickness in games and I can tell you one thing it sucks like hell
Yesterday I found an interesting article about this subject here:http://www.loonygames.com/content/1.2/feat/
The article talks about how more realistic games can cause motion sickness. Although I suffer from it very much, for me the motion sickness doesn't seem to depend on the realistics of a game...
Now to some of my expiriences:
The worst games in my opinion were the old Id games like doom1, doom2 and hexen (I didn't play them all and didn't play them long, you know why... ;-))
The worst thing is watching someone play these games, it makes me puke after 5 minutes.
I don't know if that's normal but I can normally play any 3rd person games without any problems at all. The problems are there only with 1st person games.
And even in them I can notice some differences. Outdoor-games normally don't make me sick but as soon as it gets indoors it gets bad very soon.
Surpringly the newer Id games (Quake3, RTCW) aren't that bad.
Somehow I found out, that it depends on the rotating the game makes (rotating left-right, looking up-down).
Only moving forward or strifing works very well but as soon as rotating comes into play it gets worse. Somehow I think that Quake3 and RTCW (which I could play for hours without any probs) has a different rotating feeling (look especially in the edges maybe you can see the differences I mean, which are perhaps only in my head ;-)) than Half-Life, Deus Ex, System Shock
2 or Far Cry (which made me sick after < 5 min)
I don't know if anyone could share my expiriences...