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.
lol what do exaclty mean by gaming experience. if you mean EAX< its pretty dead now, unless OP plays pre2005 games. soundqaulity qise, i would think the asus has better sound quality, plus the x-fi processors FOR processing is now pointless, since the days of vista and over- sound is always processed using the CPU.