A good thing to do if you want to get in to using voice coms with the PC is to, if you have a 3rd party sound card and a second built in soundcard on your motherboard. Use the main soundcard for a 5.1 speaker system (or your preference of speaker configuration) and then use the onboard soundcard linked to a one ear peace headset and bang you have a configuration just as good as Xbox Lives voice com setup.
Teamspeak - easy to setup and configure, sound quality can be a bit rubbish at times but generally my preferred voip client.
Ventrillo - a pain to setup and configure if you have never used but good quality.
I also use xfire for my game tracking etc