Upgrade the rear shock on my bike?
Any shock with an eye to eye length of 20.0 cm (7.9 in) and 5.0cm (2.0in) stroke will fit as a direct replacement for your shock. All the shocks available as OEM options on the 2005 Prohphet used these dimensions.

Any different eye to eye length will alter the geometry of your bike. Any different stroke length will either reduce travel, or if risk damaging your frame if the travel is too long. 

As to setup, that will depend on your weight and riding style. I will add that the prophet is a single pivot design, so you may benefit from platform technology to prevent bobbing.