I'm new to pc recording. I was noticing a huge diffrence in the quality of sound between my motu pci 424 card and the sound card that came with the asus motherboard. The 424 is much fatter/fuller sounding. Does anyone know if  this is common? Or do you think I might have a problem with the asus sound card. I ran a motu pci 324 card on my mac and I don't remember their being a big diffrence in the quality of sound from the computer to the recording cards. If you have any thoughts on this, please post, cause I'm a little curious.

I am not a techy, but i know that cpu capability and pentium processor has a say it that! if you have pentium 4, your original sound card should sound just as great (as far as i know lol)

The motu 424 pci card is supposed to be a pretty damn good sound card from the Motu stable, I can't see there being too much of a problem with it. 
That said, I changed soundcards in February this year (from an inexpensive Creative Audio to a mid-range M-Audio) and there was a marked difference in that sound too.  It didn't present a problem, but it did change the sound - I had to turn the sub on my speakers down a fair amount to compensate for the initial richer, fatter sound.  This seems to be just what you're talking about, so you may well find that it simply the soundcard characteristics matched with your speaker set-up that is the cause of your fat sound.
As your original post is over a month old now, you've probably sorted this all out.  But I thought it worth a comment.