Take a look at the creature parts; you only need to keep the best of each type. I kept a list of my current top part stats handy so I could sell any lesser versions before talking to the necromancer, so I wouldn't clutter up the part list.
You can also get rid of potion and enchantment formula by talking to the alchemist and enchanter (any left in your inventory after going into the alchemy or enchantment functions will be duplicates, and can be sold).
Alchemy plants and enchantment gems and minerals are easy to acquire after getting the battle tower, so you can get rid of any you are not going to use. Keep malachite gems, since there is a limited supply in the game, and ginseng (etc) for healing potions, but if you have multiple stacks of the same item, you can safely get rid of the extra stacks.
You can only enter the temple once, but the treasure is not that great anyway. Ragnar should mention loot and the key if you mindread him in the bandit camp (near the freed prisoner that you can trade with) before entering the temple, which spawns the key on the dragon statue's left horn (after climbing up the statue, walk forward until you get to the first wooden plank, turn right, take a step or two over to the horn and look down).
Jagon, inside the temple, thinks of a reward from Lord Laiken for the head of a slayer.
