What's the max dosage for glucosamine and chondrotin? Is there any difference in therapeutic effect at higher dosages? Any other supplements for arth?

No max. Glucosamine and chondrotin sulfate are not drugs or a vitamin, as such there is no maximum. Usually it is derived from fish so if you have an allergy to seafood or sulfates you should not take it. Most studies use at least 500mg of each three times/week. "arthritis today" has a supplement guide for specific supplements. http://www.arthritistoday.org/treatments/supplement-guide/index.php.