The DAC is a major factor in sound quality. As you go up in sound quality for a sound card, the part that really gives the boost is the DAC, but consumer sound cards don't have very good DACs, as they are really just intended for listening to mp3s, playing games, and maybe watching moves. Not critical listening. While "pro" sound cards will have better playback quality than consumer cards, the major differences are in recording quality, as that is basically what pro cards are for. Getting a nice external DAC will have much better playback than any sound card.
Hmmmm.... A good tube DAC will be hard to find at that price, unless you would be willing to mod it. Here is the best I could find:
http://pacificvalve.us/LTDAC602.html
It will be a million times better than any sound card. Forget about the price. However, it is said to have somewhat rolled off treble, pacific valve also sells a modified version to address this but it is more expensive. Nevertheless, it will still blast any sound card out of the water even unmodded.
If you are willing to go solid state there are offerings that would fit much better into your pricerange.