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...

Well, the DX wins this matchup easliy, as the DS is really a budget model.

Xonar D1 < Xonar DS < Xonar DX < Xonar D2/D2X < Xonar STX Essence