I'd use the Realtek 850 myself, it has-
EAX? 1.0 & 2.0 compatible
Direct Sound 3D? compatible
A3D? compatible
I3DL2 compatible
HRTF 3D positional audio
Sensaura? 3D Enhancement (optional)
The Audigy is only capable of 16 bits/48kHz like the realtek in analog is because that's what the audio chip is capable of. That means the 24bit/96kHz DACs capabilities on the Audigy go unused and claims of 24bit/96kHz output are consequently just a marketing scam. The ALC850 can provide up to 96khz output via S/PDIF so it's superior in that respect.
As to the Audigy using less CPU cycles, I doubt that will be the case. If you have a look
here you'll see the Audigy actually shows similar performance to an old implementation of the ALC650. As long as your board didn't completely cheese on the way they used the 850 Codec it should overall be superior to the old Audigy and you don't give up EAX support, you just don't get 3.0 support.
Anywho, it's your call, but I wouldn't use up a pci slot on that old card when the on-board should be better overall based on specs.