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I'll answer the second question. If your iMac has a Core Duo CPU it's permanently stuck at 10.6.8 or a version of Lion modified to run on 32-bit. If it has a Core 2 Duo it officially maxes out at 10.7.5, although it is possible to install newer versions like Yosemite on it with a bit of work.
 
Sims 3 works fine in my old Mac Pro under both Mavericks or Yosemite. So, New OSX is not a problem at this moment.

But, in Yosemite, one of the graphic setting may cause issues, most likely driver's problem. So, I will say it's possible to run Sims 3 in your iMac, but you have to find out the optimum graphic settings for difference OSX.

However, I am sure you won't expect high performance with those hardware.
 
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