I have a Sound Blaster X-Fi.
This sound card has destroyed every other computer sound card, internal or external that I have ever listened to.
All of the solder joints that count are soldered using a gold based solder as opposed to a lead-tin type solder.
The sound processor actually is powerful enough to need a heatsink.
The fidelity is amazing. On my system, see signature, it really made an impressive difference.
With regards to a USB based sound card, they lack in performance because of the lousy interrupt request (IRQ) that they are assigned. When the PC is asked to perform a more strenuous(sp??) task, if need be, the USB sound card gets placed on the back burner until it has the processor time to finish what it was doing with that sound file.
IMHO a PCI based sound card is the way to go.
The Creative X-Fi has an onboard DAC, as well as a ADC, and is also compatible with a single digital I/O for the fancier digital computer speaker systems/recievers.