Well I would hop on Newegg or your preffered and type in X-fi and get the best you can afford creative is not the only people that use the X-fi processor anymore and there are good choices I have the Fatality champian hooked up to my Z5500 love it over kill sound card I know now but still love it
I would recommend the ASUS Xonar DX (PCIe) or Xonar D1 (PCI).
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.