I recommend going with an external USB soundcard. I have not seen an internal only soundcard that matches the audio quality of even a cheap external soundcard. Unlike the soundcards I recommend below, professional soundcards are not usually USB devices because the latency going through USB is a problem for them.
$75 - Soundblaster Extigy - It was one of the first USB soundcards on the market. Get one used on ebay. (has a remote control)
$125 - Soundblaster Audigy NX - New USB soundcard from Creative. I haven't listened to it yet, but by the specs it should be a bump up in quality from the Extigy. Get one new on ebay or via froogle. (has a remote control too)
$160 - M-Audio Audiophile USB - Get one new via ebay. (big step up in audio quality, but no remote control)
$300 to $???? - the best sound will come from using a soundcard with a digital output, plus an outboard digital-to-analog converter (DAC) - The soundcard I recommend with a digital out is the PCI version of the M-Audio Audiophile, for reasons I will explain below. Get one used for $90 on ebay. Like professional soundcards, the DAC will cost as much as you care to spend. Starting at about $200 you will see a clear improvement over the USB soundcards. I would look for a DAC in the classifieds of Audiogon.com. For buying high-end audio components Audiogon is awesome. It has more sellers than ebay, and it's not an auction so you are more likely to get a great deal if you send in a quick offer. Some recommendations on a DAC: For a little over $200 you can get an MSB Link DAC III. That will sound way better than any consumer soundcard. If you want to improve on that, for another $200 you can add a Monarchy DIP to the MSB Link DAC, or buy MSB's upgraded power supply. The downside of this approach (other than the cost of the DAC) is that if you want to digitize analog input, you will have to buy another separate piece of equipment.
A word of warning, many soundcards have a digital out, but not all of them are what you want to hook up to an external DAC. For example the Extigy has a digital out, but it will only output at 48kHz or 96kHz. That means for CD (or mp3) audio, which is at 44.1kHz, it has to resample the signal. This creates some artifacts. Going from 44.1kHz to 48kHz is kind of like scaling up a digital image by a fractional value, at best the picture will end up a bit blurry. It is really hard to find out if a soundcard does this from a manufacturer's website. If it doesn't resample the signal they might say something like 'true bit-for-bit digital', but most of the time they won't say anything either way. I know the M-Audio Audiophile can output at multiple sample frequencies and hence doesn't resample the signal.
All of the options I mentioned will sound really good. Even the Extigy, the cheapest, will be completely free from the crackle and buzzing you describe.