Anantech recently compared CPU utilizations for soundcards. Most on-board sound took between 5% to 15% of the CPU. Most sound cards took 1%-3%.
My maths says, assuming you aren't limited by your video card, that you'll see an improvement of between 2% and 14%.
Plus, you'll get a better sound and possibly a good gameport out of the deal.
I have a regular Audigy LS and it works just great at 3 GHz, including the gameport.
-Llama