I think you wlil get better sound quality from the Asus Xonar DX (like hell_storm2004 posted), but you will have a better gaming experience on the Creative X-FI Platinum (both are pci-e).
Be warned that if you get the creative card you need to be VERY careful with drivers and be a bit technical, the software is sensitive (ie, you need to do a complete uninstall and bios disable of your onbord soundcard before install.