I am not sure what your current card's capabilities are. But if it already gives you Surround Sound and/or 5.1 and you have a descent set of speakers, then getting the Audigy won't give you much more, in my opinion. I am assuming when you state "performance" that you are talking about a sound issue and not game performance.
From your specs, doubling your RAM will give you better game performance though. Also, tweaking your system enhances game performance too. For any OS there are two sites, with links to more, that I recommend you visit for getting tips on how to make your system hum along. They are http://www.speedquide.net and http://www.pureperformance.com
Hope this answers your question. Good luck pilot.