Keep one thing in mind, an amp isn't a soundcard replacement but rather a complement, which is why I advised on both, considering the headphones you plan on getting.
sound card ? - Page 2
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a USB soundcard would work, just not as well, taking in account the purpose of the soundcard for the OP, which is why he has to go with an internal soundcard.
You should consider getting a PA2V2, it's got good reviews across the board, a nice price and no shipping issues or added expenses ever reported by anyone: http://www.electric-avenues.com/amplifiers.html
A few reviews:
Why do you want a USB soundcard if you play a lot of games, like you said on the first page? Get an internal soundcard, it is your best option for what you said you do.
Oh, you didn't mention you had a second machine and wanted to get a separate soundcard. Well, again, will you be playing games, listening to music, watching movies on that second machine?