Halo 2 (the LE at least) comes with a 2 month free trial. Since you're using a free trial I'm guessing you don't have the headset, right? I don't have Live yet and haven't tried the trial since I thought it would be pointless without the headset. How's it working out for you?
I bought an El-cheapo HIP GEAR headset for $20... it works great for now, I don't know how it compares to the official MS ($30) one though.
That said, I often play Burnout 3 (racing) on Live without the headset. I can hear the chatter through speakers, but don't feel the need to talk to anyone really.
Still, FPS on Live without a headset is probably not a good idea. I did it for a day or two but you need one to enjoy the full experience of team work and such.