Very minor improvements are possible, as it offloads some of the software stuff from the CPU to the sound card, but being honest any performance improvement, besides better audio quality and performance (ability to do more, use more effects, channels etc), are going to be very small, and noticeable ones are likely to be placebo.
That said, if you dont need the front panel, the Auzentech Prelude is the card to go for now over the Fatal1ty (coming from a current owner of the original Fatal1ty, which is meant to be one of the best of the X-Fi range, just below the Elite Pro), it has better quality components than the Creative-built series and has been shown in tests to produce a better quality output, although whether this would be noticeable to average joe is another matter, and Auzentech have shown far more potential in dealing with software issues, for example a patch for Vista and 4GB RAM, which Creative STILL haven't done.
The other issue is, even if your source (the new card) is better, you may also be limited by your 2.1 and headphones, unless its a seperates based system, a standard (ie Logitech et al) will limit the performance improvement you get from an improved card, although it would still be noticeable with extended use. If you can suffer the loss in standard bass compared to the creative/logitech etc sub jobs, a good hi-fi amp and well chosen bookshelf/floorstander speakers (reasonably priced second hand) will produce a much better sound, improving especially the mid-range and high end clarity in general. If you listen to a lot of instrumental work, with midrange, you may see as much benefit upgrading your sound system, as the card
If you have an Aego M 2.1 set, these are pretty much considered the best of the PC 2.1 setups available, so would be much less limiting than some of the equipment available, however second hand high quality seperates gear, whilst more expensive, can ultimately produce the best sounding pc setup, and are a lot more customisable to taste.
Sorry if thats a slightly extended answer to your question, but I if you have the cash available and are after improved Sound Quality as you mention, perhaps there are more avenues you can consider, although you will recieve a 'richer' sound straight away, your speakers/headphones may limit the benefit to an extent
Direct performance improvements, except in the case of heavy CPU calculations being offloaded, and improved sound effect capability, are likely to be minimal.
