iStat Menus can monitor GPU memory usage (oddly, it's under the CPU tab) When playing D3 it's always full on my HD 3000, but I'll test it on a 9400/9600GT Mac to see what it does!
Edit: Ok, this is what I found. Default, the 9600m GT (512MB dedicated RAM) defaults in the D3Prefs.txt file to Hardware Class 2, everything low and 1280x800. I played with it like that, sat in town, cmd+tab to desktop to check iStat, it was about 1/3 graphics memory usage. I then did a run from New Tristram to Old Tristram and back on Hell (Reign of the Black King) and got a solid 50+ fps. cmd+tab'd, iStat Menus showed about 1/2 graphics memory usage. I changed the settings to all high (no shadows and no antialiasing) and native resolution (1440x900). I quit D3 completely and restarted it, so the graphics memory was wiped (checked it) Did the same run again, cmd+tab, graphics memory at 1/2 usage in town then at the end of the run, 2/3. Again, it was a solid 40fps, so I quit D3.
I decided to change hardware class in the D3Prefs.txt file to "4" (leaving all the settings on high/native) and did the same. New Tristram, graphics memory was over 1/2 usage, and by the end, it was full! For most of the run, I had 30fps solid (and it was glorious!) until I hit the stuttering, just as I entered Old Tristram.
This says to me (I'm no expert on graphics/coding/etc) that the advanced spell effects of hardware class 4 aren't clearing from graphics memory and are causing the GPU to struggle. Meaning 'lesser' cards (like my Air's 384MB HD 3000) are getting bogged down quicker and are getting stutter from the get go (plus this could explain why changing settings makes no difference with hardware class 4!) But as S4d1k said in another thread, they've found the cause of the stutter, so I'm more than likely wrong about this!
Anyway, that's what I found, I didn't test the 9400m (256MB shared), however, I assume that would be affected quicker. I can test it if needed though!
Edited by iNeko#2151 on 6/11/2012 6:06 AM PDT