Wizard Robes are made from purple wool and gold. They have better durability than leather armor, but offer almost no protection. Their enchantibility score is higher than gold, so you get better/more enchantments per experience level spent in an enchanting table. They can be repaired on an anvil using string.
The tophat has no special properties, but it looks nice (goes well with the nonmagical wand from the Dr. Cyano’s Wonderful Wands mod).
Wizard’s Hat and Witch’s Hat
If you drink a potion while wearing a Wizard or Witch hat, the hat will absorb the power of the potion and make one of the potion effects permanent for as long as the hat is worn. The magic is permanently stored in the hat, so you can store the hat and put it back on to reapply the potion effect at a later time.