The sound card is used to create a digital signal from your microphone or other show. USB microphones go around all that because they have all the electronics inside them They deliver a good quality digital signal to the computer -- whether or not you have a sound card installed.
We keep dancing around the "best microphone" because it's too easy to get burned. There's no shortage of complaints about the Samson U01 microphone and its cousins because some tradeoffs are needed to push all that electronics processing inside that one little microphone.
The microphone element, the preamplifier, the processor and digitizer are all smashed inside a USB microphone. The more expensive microphones split things up and you can choose the quality much more accurately.
Take this for one example...
http://www.kozco.com/tech/audacity/pix/TimPlays.jpg
The microphone is a very high quality analog unit and presents its tiny, low level show on an XLR connector.
http://en.wikipedia.org/wiki/XLR_connector
This plugs into a sound mixer...
http://www.sweetwater.com/store/detail/PV6/
... which amplifies and mixes the show with as many as three other microphones. It produces a very healthy, powerful analog signal which I plug into my Mac which has a very high quality analog to digital converter. The whole thing is recorded by Audacity.
So that's one product line. The mixer can be split up into the Preamp (typical -- the Arts unit) and you may or may not need the rest of the mixer knobs and controls at all. That mixer can be bought with a digitizer inside and it will make the digital bitstream all built-in. That's the USB version.
http://www.sweetwater.com/store/detail/PV6USB/
USB microphones pack all that into one microphone, the disadvantage being you can't change anything -- you can't reach over and turn up the volume a little. Setting microphone volume is impossible on some models.
Our company, on the other hand, has two Blue Snowballs and they work pretty well. I will shortly do a voice test into the Logitech unit. That works OK, too. USB microphones are very simple, cheap, dead easy to use and are indicated if they fit your tasks.
For good versatility and top quality, you need the individual pieces model where you can choose each piece.
I love this microphone for my voice, that preamp is really quite and well behaved, I need to mix four microphones into a show and since I have a Mac, I don't worry about the digitizer at all. Your mileage may vary, void where prohibited, licensed drivers only.
http://www.kozco.com/tech/audacity/clips/timWed_A.wav
Koz