Before plopping down a wad of dough on a new sound card, it would be a good idea to run some tests on the one you're using. Most of the onboard sound cards that are currently being used with newer computers are really pretty good. Keep in mind that the sound you get out of the computer is only as good as the sound that's going in. I suggest bypassing the mixer and going directly into the computer's line in from the keyboard. Then put a high-quality midi file in the keyboard, record it dry (no effects) on the computer, burn it to a CD, then play it back using your home or car stereo system. You may be pleasantly surprised.
Good Luck,
Gary
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PSR-S950, TC Helicon Harmony-M, Digitech VR, Samson Q7, Sennheiser E855, Custom Console, and lots of other silly stuff!
K+E=W (Knowledge Plus Experience = Wisdom.)