I would guess the best results would be around mid or low settings.
With the changes made to the graphics engine through patches over the course of the last 2 years, the game has been made very much multi-core aware and as such the performance of the game improves as the physical core count increases. It's been suggested by Blizzard Tech Support that the game supports all current and future multi-core processors regardless of physical core count and that performance will scale with physical core count. Notice I said "physical" core count, as HyperThreading can actually cause performance loss....