I have a combo of two Audigy 1 cards, SB0160 and SB0090 - first running Creative drivers and the second kX Project drivers. The first card is used as the default output port for surround sound in games and movies and for MIDI playback with multichannel effects. The second card accepts 6-channel digital signal from the first, converts 5.1 to 4.0 using a custom matrix and produces two slightly different versions of the signal for rear channels (used by different room correction presets on next device in the chain). This card also applies a custom 2-to-4-channel upmixing algorithm to stereo sound and adds reverb when Audigy Sw Synth is used on the first card (that synth is software-based and cannot be processed by the card's own DSP).
P.S. Since a lot of people replied about using an X-Fi, here is a file I made to port effect presets from Audigy control panel to X-Fi audio console. Replace the .dat file (after backing up the original if you saved your own presets earlier) in C:\ProgramData\Creative\CADI\Preset\ in modern Windows or C:\Documents and Settings\All Users\Application Data\Creative\CADI\Preset in XP, reboot, launch the console in Audio Creation Mode and check the preset list for Reverb, for example - it is now a lot longer than previously. Hope you have fun!