Good sound cards in my opinion are made by companies that are specialized in these things.
M-audio, ESI, EMU, Apogee, Tascam for instance.
I love my ESI Juli@. Cost a bit more than 50 bucks though. But there is some nice cards ranging from 50 to 80 bucks.
Below that you just get the usual PC hardware brands (indeed Asus, Creative etc.). I wouldn't recommend those if you want to get quality sound.
I prefer PCI above external (external cards are often a bit buggy).
24 bit is sort of useless when you plan on listening CD quality or mp3's.
Most recordings are done in 16 bit, so most rips are done to 16 bit. I wouldn't bother if I were you
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What headphones are you using? Without trying to bring you down I might tell you whether they are worth a good soundcard
And then there's this: when you buy a defective Minidisc, DCC or DAT recorder that still wants to switch to rec mode for a couple of bucks, you can use this as an digital/analog converter (DAC).
You can hook up pretty much every soundcard that has coaxial or toslink digital out to the md/dcc/dat and it will provide you with a sound that's way better than this card would normally do. Most of the time there's a better headphone preamp built into the MD/DCC/DAT than there is on your soundcard.
This is how I used to listen to music on my PC before I got the ESI.
Also remember: Most serious sound cards don't have headphone jacks. They have sperate rca or mono jack (not mini jack) in/outputs.