I would recommend the ASUS Xonar DX
(PCIe) or Xonar D1
Both offer excellent sound quality and a full feature package.
For your best sound quality, use the analogue connections as these cards create a better analogue signal than the z5500's onboard DAC.
Stay away from Creative's offerings, they are extremely subpar and overpriced.
HT|Omega and Auzentech also both offer high quality cards and anything from either manufacturer is a ready alternative.
I would however recommend sticking with the C-Media CMI8788 (AKA Oxygen HD) chipset, the same as in all the Xonar cards.
It is the newest offering from C-Media and offers an excellent feature package along with high quality sound.