You'll need: an audio interface with built-in mic preamplifiers, a mic, cables, headphones, accoustic treatment, and a quiet room.
Sound Blaster products are great gaming cards, but you'll really need a good pro interface. There are numerous products on the market that'll get the job done, and some are definitely better than others. Essentially, you'll want a stereo audio interface (USB or Firewire) with High-quality mic preamps and a decent A-D converter. If you're handy with Protools, you may want to look at on of their M-powered products or an Mbox2. I've had an Mbox (first generation) for years, and the newer models are definitely an improvement.
Scour the forums, and you'll find plenty of mic recomendations for voiceover and narration. Just be certain that if your mic requires +48V phantom power, your audio interface can supply it.
The quiet room can be a bit of a challenge, since your computer will probably be kicking out a fair bit of fan noise. If possible, run long cables (keyboard, mouse, monitor, audio interface) and hide the PC in a closet (take care it doesn't overheat) or another room. A few pieces of judiciously placed accoustic foam (or even heavy quilts), and you can keep room reflections under control.
I would suggest the new Digidesign Mbox 2 Pro. It is a firewire interface, not USB, so the latency issues with the original Mbox and Mbox 2 shouldn't be a problem. I have the Mbox 2 (USB) and the latency issues with monitoring drive me batty. I use mine as a portable recording system for recording v/o with Dick Vitale. I've recorded v/o for Sprint, EA Sports, Scott's Lawn and others using this system. I generally use either an Audio Technica 4050 condenser or my Schoeps MK-41 mic for the job.
Check this page to see if your system meets the requirements to run ProTools LE:
I'm going to assume you want to record audio into your computer, not from it.
You need a good mic. An audio interface with a mic preamp. If you plan on a condenser mic, your interface will need to supply Phantom Power.
As Stephen also states, you need a nice quiet room with good acoustics.
I suggest firewire interfaces rather than USB interfaces, with the exception of the Rode Podcaster mic which has its own headphone jack. You may already have software that records audio. You can plug the Podcaster USB mic in and be on your way.
I have a more expensive and exotic rig, but it costs quite a bit more.
A simple Pro Tools LE system should do.
For more complicated audio, you should consider haveing two drives. One for system and applications and the other for media files.