OK, lets do some comparison here. These are my own opinions based on several different cards.
My wife has a Santa Cruz Turtle Beach soundcard with Cambridge Soundworks 4.1 speakers. My opinion is that the sound is a tad muffled. Its in a Dell system.
My son has a Soundblaster Audigy 2 with Monsoon 5.1 speakers. The sound is awsome. Its in a home built machine.
My other son runs a Soundblaster 5.1 Platinum with Cambridge Soundworks 5.1 and I think the Audigy 2 sounds better. This is in a home built machine.
I have an HP kick around system with a Soundblaster 5.1 and when it had good speakers it sounded darn good. Just a bit less than the Audigy 2.
My home built system has a Soundblaster Audigy MP3+ soundcard with Klipsch 5.1 speakers and it sounds as good as the Audigy 2.
Obviously, this is very unscientific. Different speakers connected to the various cards, as well as I am judging by what my ears like to hear. But, for my money, I would get either an Audigy or an Audigy 2.
Lastly, be aware of what plyduster mentioned. Some of these cards have compatibility problems with some systems. This includes the Audigy 2. So, whereever you purchase the card, be aware of their return policy just in case.
Good luck!