Since you are going from Altec Lansing VS4121 to a Logitech Z2300, this alone you will see a huge difference.
Altec Lansing VS4121
Total RMS is 31 Watts
6 Watts per speaker
19 Watts Sub
Logitech Z2300
Total RMS is 200 Watts
40 Watts per speaker
120 Watts Sub
In a general answer, the X-Fi is a better sound card then the Audigy. You now have to ask yourself is the extra cost worth it?
Here was a review done by Extremetech. The review is from 2005 but it gives you a bases to help you make a decision.
http://www.extremetech.com/article2/...1813855,00.asp
