If I'd have to bet I'd say it's the inner ear. Somehow you take in a lot more vocal info from your surroundings while playing and the headphones distort that flow of information for the brain. You get sick because that's conflicting with your other senses. Those other cures are most likely placebo and stuff that's the most soothing for the senses.
Hopefully over time you'll get accustomed to such-and-such games and won't need all this crap to play them.
Thanks, all sounds about right :) And yes, the idea is just to use whatever I can to be able to play for long enough to stand a chance of getting used to it. I can now play Mass Effect for extended periods of time, but Painkiller still destroys me pretty quickly. Precursors seems a little better, but that's because all I do is stand there and die. I also got Far Cry but haven't gotten around to downloading it yet :)