Sounds like a processor issue to me. I have a hunch that a GF2 would run that game VERY fast, with at LEAST 50 players. The player models in that game (I have heard anyway), arent extrememly detailed, so the number of players shouldn't affect speed a whole lot.
What vid card do you have? You said something about resolution boosts. Maybe you "Boosted" too much? What kind of internet connection? If you are choppy ALL the time (Like even when there is only 1 or 2 people, it probably your processor, or too high of resolution. If it is only choppy when there are a lot of people, I would blame the Internet connection. Right when a level starts, go somewhere where you can get to a big hill, without looking around while you are going there. When you get there, turn 360 degrees. If it is choppy the first time you turn around, and not as choppy or not choppy at all the second time, I would blame memory bandwidth or amount.