I've been playing marathon session os Vice city so that I could eventually justify buying San Andeas and recently discovered that do to an incredibly frustrating camera I am indeed getting motion sickness from it while running though the streets so I'll discribe what it feels like in detail for you:
It starts with stomach twinges or nausia, you might feel like you're having hunger pains or maybe gas maybe worse than this maybe less but it will always start in your gut. Next you may start getting slight headaches or an instinctual desire not to move around so much like there's presure on your eyes and a fast change of direction makes you flinch, maybe you'll find yourself trying to stare at the screen harder than normal (which probably is where the headache comes from) lastly you'll start feeling fatigued or exhausted like you want to lay down at this point you are officially Motion sick.
The only cure is taking medicines like Meclazine BEFORE you expose yourself to a motion sickness inducing experience. because afterwards all you can do is lay down and close your eyes for and hour, let the exhaustion and nausia pass and hope you arnt so far gone that you really need to throw up... avoid dairy products... trust me.
Usually this happens about 2 hours or so into GTA:VC for me, but easily 15 minutes to a half hour in Timesplitters, Doom, Rainbox Six or any other moderately high paced first person shooter which requires lots of quick movment and perspective changing. I've been able to last a good 3 hours on VC Metriod prine i can go about 2 and a half with lots of morphball use, but it gets me eventually.