Rainbow Six 3 is really great. Don't listen to ferill, though.
(The SA-80 is that .50 cal sniper rifle, right? That weapon is so unbalanced it takes NO skill.) Learn to use the G36C, Desert Eagle, and frag grendades effectively. You'll be unstoppable. Well... that combo always worked for me, at least.
Return to Castle Wolfenstein was also a great game. Lotsa' good ole' drama and teamwork. Looking back, this game was the one I had the strongest addiction to, following Unreal Championship.
I got tired of Mechassault. I sold it.