Edirol FA-66 Firewire Interface

Greetings,
As I used do a bit of casual home recording, I own an Edirol FA-66 Firewire Audio interface that is currently connected to my Dell desktop. Routed through a Mackie mixer, old 2-channel power amp, to speakers.

http://www.roland.com/products/en/FA-66/index.html

As you can see, http://www.sweetwater.com/store/detail/FA66/, this is a pretty inexpensive piece of equipment.

Would I see a significant improvement in audio quality if I used something like a Cambridge DacMagic or Benchmark DAC instead?

Also, since I have already invested in a Firewire card for my PC, if I do buy a new DAC... USB or Firewire?

Thanks very much in advance.