Resistance gets capped at 75% reduction in damage, somewhere near 500 resistance. I have stacked it to 480, and it gives me around 74% reduction in damage.
Note that resistance does NOT work like resilience. Resilience reduces damage taken from other players and their pets by a flat percentage. Resistance, on the other hand, gives you a percentage of how many of your opponents spells you will fully resist. Right now, I have a 74% chance to fully resist a frost spell. I have a 26% chance to partially resist, or not resist at all, a frost spell.