There seems to be 2 different kinds of Treasure Chests in DeathSpank:
- The first kind is the grey/silver looking chest that can be opened with no requirement and drop lesser items leveled to the area you are in (e.g. if you are in a low level area where Level 4-5 enemies roam then the chest will drop lesser weapons/armor/potions/food for around that level)
- The second kind is the gold looking chests that require a Gold Key to unlock. The keys can be acquired through quests and seem to come as a rare drop from certain boss like enemies. The gold chests drop a lot more and rarer items compared to the normal chests. These chests are also leveled to the area you open them in however. So if you have a key and you're making a return trip to a lower level area to complete a side quest or something try not to waste a key in a gold chest!
The gold treasure chests actually have set loot that will always drop from them no matter when you open them. For instance, the chest in "ogre land" behind the trash heap will always have the best frost hammer and +220 health armor.