As Dave said it really does depend on what you're recording.
However, pretty much regardless of what you're recording you're going to need a powered condenser mic, your soundcard is unlikely to have phantom power so unless you do upgrade your soundcard you're unlikely to get good sound.
You can get USB condenser mics but these are really only good for spoken voice.
Personally I have an Alesis Firewire 12 track desk which allows me to record 12 channels, 4 of which are powered (it basically works as an external sound card as well as being a pretty good mixing desk), and because it's firewire you only do A/D once and the rest goes straight onto the computer. http://www.guitarampkeyboard.com/en/69216
It's the thing on the right and it's only costs about £200
However, pretty much regardless of what you're recording you're going to need a powered condenser mic, your soundcard is unlikely to have phantom power so unless you do upgrade your soundcard you're unlikely to get good sound.
You can get USB condenser mics but these are really only good for spoken voice.
Personally I have an Alesis Firewire 12 track desk which allows me to record 12 channels, 4 of which are powered (it basically works as an external sound card as well as being a pretty good mixing desk), and because it's firewire you only do A/D once and the rest goes straight onto the computer. http://www.guitarampkeyboard.com/en/69216
It's the thing on the right and it's only costs about £200