I believe that Snowtech magazine did an article on this exact thing - what is best for an 800 Polaris motor - call them or email them, and they hooked me up with what they suggested but by my recollection the items were:
SLP Flow Rites (2-2")
SLP High Flow Intake
SLP single pipe with can
Boyesen RAD valves (or equivalent, choose your brand)
Make the required clutching and jetting changes and you're good to go.
If I remember right, these mods were around $800-$1200 or so, and they were good for about a 20 HP gain. These were tested by Snowtech magazine, and supposedly the mods were very forgiving for elevation and temp changes.
Again, call or email Snowtech, and they can either tell you, or can hook you up with an archived magazine article that specifically outlines what they did.
Also, SLP can provide tuning recommendations for this exact setup.
Good luck!