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Depends on if you're doing the work or if you're going to have someone else do it.
I assume by the question you're going to have someone else do it.
Consider the cost of parts and labor. It's probably not worth it to convert from auto to manual.
The basics are that you'll need:
A manual peddle assembly
A T56
A C5 shifter assembly
Clutch/flywheel and all associated parts
Master/Slave cylinder + lines.
(possibly different torque tube)
3.42 or bigger gears.
You'll need to cut into the firewall I believe, although I've never seen how the auto->manual swap is done on a C5. You'll probably need to cut into the center console to open up a place to mount the shifter.
Sounds like it'd be better to sell the auto and buy a M6.