If you plan to get good, and join a team or "clan", you'll need voice. You'll also need a headset to go with your microphone, otherwise everyone will hear your speakers over the microphone, meaning not only your game sounds but also their own voices will be played back to them the whole time.
The days of trying to type messages while being shot at are over. Even keybinds are obsolete. Now any clan that seriously expects to compete is on voice. You can just yell into your mic "I'm waiting under the steps in the middle, tell me when to go!" and your buddies can tell you how many enemies are coming and exactly where they are. Much better than "need help kthx" from some undisclosed location.
And now games are coming with Voice built right in. In UT2k4, you join the server and the voice channel at the same time, and are automatically talking to your teammates, and can even talk to people on the other team. I find that annoying though. I never talk in pubs, and my team uses Teamspeak to communicate, although I'd prefer Ventrilo.