Turn these souls into Knight's Honor weapons now, at 5,000 souls each: the Moonlight Butterfly's (Crystal Ring Shield, from any shield at +10) and Sif's (Greatsword of Artorias, from the Straight Sword Hilt every class starts with, at +10). If the Core of an Iron Golem or the souls of Quelaag, Ornstein or Priscilla are still in your pack, make the Golem Axe, Chaos Blade, Dragonslayer Spear or Lifehunt Scythe as well. Keep the rest: a Soul of Smough (if he fell last) waits for Smough's Hammer in the next run. Never use a boss soul for souls: each run gives one, and one soul makes one weapon.