My recommendations have not changed much. Basically comes down to budget/needs more then anything.
Low End:
ASUS Xonar DG
Budget:
ASUS Xonar DS
Mid-Range:
ASUS Xonar D1/DX
HT Omega Striker
High-End:
Auzentech Prelude
Auzentech Meridian 2g
ASUS Xonar D2/D2X
HT Omega Claro [Non-Halo line]
Top-Tier:
ASUS Essence ST/STX
Auzentech Forte
HT Omega Claro Halo [and varients]
I will say that most anything above the mid-range will probably be a little limited by the Z5500's [good solid speaker set, but not high-end]. On a pure price/feature basis, the ASUS Xonar D1/DX and HT Omega Striker really stand out, and are the two cards I would look at first.