Is this a soundcard issue?(music production)

I recently built this system:

Q6600 @ 2.4 gHz
2 GB ram
8800 GTS 640 mb
Vista 32


I use Reason 4.0 with a midi keyboard and Korg Padkontrol drum machine to make my music, and compared to my old pc it sounds very thin, low volume, and muddy.

I have not bought a professional audio soundcard, but would that help the overall sound or just latency issues?

If you have no soundcard but the built in one on the mb it could very well be the sound card, do you have similar problems with playing back music in general on the system? if so it probably is the sound card, remember the weakest link in an audio chain decides the overall level and accuracy of reproduction, so it could also be a dodgy lead or crap speakers or amp. If your only problem is bad sound through reason (as opposed to while playing a cd in media player or something) then it is probably a driver or hardware/software settings.
If you want to produce music on your computer you really should use a dedicated soundcard and a decent (and reletively flat response) speaker and amp anyway.