1) Never EVER leave sql server max memory setting at the default.

2) For 32GB system with JUST sql server running I would start at 27GB and monitor for execessive paging and go up or down from there. The more things you have running the lower you will need to drop the memory. Remember, max mem setting just controls the sql server BUFFER POOL - there are MANY other buckets of memory sql will use that are NOT constrained by that number.