I'd definitely agree with Jon that America's Army rocks. I haven't played in ages, but that was partly from burnout, partly from work.
It's unreal engine based, so if can run UT2003, you can easily handle America's Army.
The stats tracking system is great, as it prevents griefing by teammates, so you can jump on a server with a lot of people you don't know and still not have to worry about a teammate ruining the game for everyone. Occasionally you get screwed for Honor Points when some dummy sits on your grenade, but that's just part of the game
Fable is probably the biggest disappointment in gaming I've had this year. I was expecting a lot more. I guess I should give it a couple more hours, but I really hate the static world and mission system. And the stupid cards that you get to buy physical appearance. I wanted an immersive RPG system, and instead it's paper-doll FedEx Barbie.