Here is an interesting thing that I ran into, I upgraded from my AMD 6970 which was getting around 80-100 fps in D3, to two 670's, and whether I SLI them or not i get a good amount (90ish) but then once i see action on the screen it dips WAY down (10ish), and stutters there for awhile until the fighting is over. I am a little confused considering I can run everything three times faster now than my 6970 except for D3? any ideas?
It's known at this point that many users have an issue with nVidia drivers and D3 not playing well together. If other games do not exhibit any problem, and it's only D3 acting like this, wait for D3 patch or next nVidia drivers.
Hope it's a driver issue as well. Just built brand new gaming rig with one 670 2GB (Asus) with plans to SLI soon. Getting 100FPS @ 2560x1440 with v-sync off. But with v-sync on it averages 60fps as it should but drops to 30fps (!!) during some big battles. Very annoying on such a beastly card for this game. Other games just haul on the 670 though so I have hope. Very disappointed but I think D3 is big enough for Nvidia to care. Same goes for Blizzard.