Well, having had an X-Fi, as well as most other Creative cards, and now listening to things on my board's ALC889 onboard, yes, there is a difference, but it's not that vast any more. Yes, I prefer the warmer X-Fi sound, but for casual listening or gaming it's VERY capable. It is all down to the codec, though.
Xonar is a better music card, X-Fi is the better gaming card. Choose accordingly.
Another +1 for Pro vs Ultimate. Just not enough there to justify, unless it's an "I want it all" mentality.
Of course, there's always the spanner in the works of suggesting going Phenom II for the fact there's practically no gaming difference...