Resistances in D3

[MongoJerry]

I think LucityAxel at the Amazon Basin says it succinctly:

Damage reduction from Armor and DR from Resistances are calculated independently, so the most efficient way to maximize DR from gear is to have an equal contribution from both:

10 Armor (or 10 Strength) = 1 Resist All (or 10 Intelligence)

This means you ideally want to keep your Resistances at roughly 10% of your Armor (aka a 10:1 ratio). You will get more bang for your buck by increasing whichever of the two is lowest.