Use this to remove all of your Realtek and Creative driver remnants:
Guru3D - Driver Sweeper (Setup) download from Guru3D.com
Reboot then load the realtek driver. Bubba is right, make sure you have the correct Realtek driver (I believe the X48 is HD Audio) and try it. The X-Fi has had a number of problems with Vista64 and 4GB+ of RAM but I believe that has been limited to nvidia mobos. I know I had that problem with an X-Fi and an nForce4 (A8N32) mobo and had to pull it. I use an Auzen X-Fi Prelude now with a 780i mobo and 8GB of RAM and it works perfectly. If you use the X-Fi, I would get the latest drivers on their website before you install the card. See if we can get the Realtek to work first, though.