I also extremly enjoyed Freedom Force, but I also found it to get rather choppy ater a while playing. I have an 867 G4, 512 RAM, and a Radeon 9000. I found that after playing for a level or two I had to relaunch the game and it would improve gameplay. Besides that I don't know.
Than again, I'm having issues playing BloodRayne on my G4, it gets rather laggy in odd places, at the lowest settings. May be its my computer.
Than again, my G4 is in the shop due to a directory issue with the hard drive, which would explain the issues I'm having. May be that was the problem all along. Anyhow, just be sure you have all the little apps runnning in the background turned off. Things like Plamdesktop have little mini-aps running that take up speed, and also shareware programs like Synergy tend to spike every 5 seconds or so. Turning them off helps. (BTW Synergy is the best $5 you'll ever spend)
Also, thanks for the skins info, I'll have to try them out when I have some free time.