It's probably both....
Been playing on a 2.4 08 Macbook Pro with the 256MB 8600.
Have not played much on the "mac" side as I figured, and rightly so, that I would get much better performance and graphics on my Windows 7 partition under bootcamp.
Was playing a bit of a marathon session the other night, 2+ hours or so, and had a sudden "blink out" with no warning. Game just went "poof". No dialogue
boxes or anything....no "windows" explanation, so it leads me to believe that it was graphic card related and possibly an overheat issue. (Frustrating as I had not "saved" in quite some time, LOL)
The comp was getting pretty warm, but nowhere NEAR the levels of heat that WoW typically puts out, so it may be Sims 3 specific.
As a side note, my daughter is ALSO playing Sims3 on a white MacBook w/4GB ram and the old X3100 Intel integrated video. IMO, and hers, the game was nearly unplayable on the mac side of things, but it is highly playable on the the WinXP partition she has under bootcamp. (I did download and install the latest X3100 drivers as well, though)
She lags when she has an in game "party" but seems to be pretty smooth around "medium graphic settings at native resolution.
Hope this helps.
WRC