Need help on sound card!!Can anyone help pls...

Hi all!! I feel the on board sound card is not very nice for me. So i going to buy a sound card, but i am in dilemma on choosing the sound card. Whether ASUS XONAR DS (PCI) or ASUS XONAR DX( PCI-E), or you guy have any other suggestion? What different between a sound card put in PCI slot and PCIE slot? I wonder which sound card provide a better quality of sound? I always watch HD movie and listen good quality of music. So i need a sound card that provide me very well quality.

Beside that, i saw the 2 sound card dimension is quite long. I using Gigabyte Ep45-UD3L. I wonder my board can fit the sound card or not, because i have 1 gtx275 in my com which is already taken 2 slot space. I worrying the sound card will touch my RAM or the RAM slot.

Can anyone figure out for me? Thx for advance...