OP, you may be better to stick with onboard sound, and install a video card that takes up both slots. A single slot card especially a higher end one, is going to generate a lot of heat in that small case. It may cover up your other slot, but having a highend card that exhausts the heat out the the back of the case to me would be much preffered over a add in sound card that is going to give you only marginally better sound. I used to have an X-FI, I gave it away because the sound wasn't that much better over my Gigabytes onboard, it simply wasn't in my opinion worth re-installing into latest build.
Just my 2 cents, others will likely disagree.