I strongly recommend a headset rather than a separate microphone, for use with Teamspeak. If you have a desktop microphone, it's likely to sound tinny and/or echoey to the people you're talking to and you'll find yourself repeating yourself frequently.
If you're going to use Teamspeak for any extended period of time, I recommend the Plantronics Audio90 headset. It's larger than your average walkman-style headset, but it's extraordinarily well designed, sturdy in the main part, and VERY light on the head.. no pressure on your ears.. you can wear them all day and forget that they're on. In fact the biggest problem with wearing this headset is that you DO forget you're wearing them, and get half strangled when you walk away from your desk.
They're pretty good value, at between £15 (Ebay) and £24 (PCWorld), with a really good frequency response range, quite well suited for general audio/music too.