Sound Card - PCI-E (1x) - For listening not recording

My new PC's onboard sound is very noisy compared to my M-Audio Delta44.  It's fine for casual listening with speakers, but awful with headphones.  However, I don't want to use my Delta44 for music listening, because it's connected to studio monitors rather than my surround-sound speakers.

I'd therefore like to buy a relatively inexpensive PCI-E (1x) sound card, with 5.1 or better output.  This would be used for listening but not recording. I've read good things about ASUS cards and I'm wondering if the Xonar DX would be a good option.

Are there ever any conflicts between PCI-E (1x) sound cards and a PCI card such as the Delta44?  I'm using Win7 64, if that makes a difference, and my motherboard is an ASUS P6X58D-E.

Thanks!