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First, you can't play Fallout 3 with your iMac, because of your bad video card. I think it is possible with that MacBookPro
 
Here is few facts:

Minimum System Requirements
OS: Windows XP/Vista
Processor: Pentium 4 @ 2.4 GHz or Equivalent
Memory: 1 GB (2 GB for Vista)
Video Memory: 256 MB (nVidia GeForce 6800/ATI Radeon X850)
Sound Card: DirectX Compatible
DirectX: 9.0c
Keyboard & Mouse
DVD Rom Drive

Recommended System Requirements
OS: Windows XP/Vista
Processor: Intel Core 2 DUO
Memory: 2 GB
Video Memory: 512 MB (nVidia GeForce 8800/ATI 3800 series)
Sound Card: DirectX Compatible
DirectX: 9.0c or 10
Keyboard & Mouse
DVD Rom Drive

just change the windows things for mac things.

your iMac will not even run it or even thinking about running it smoothly.

MacBook Pro will run it I'm quite sure (it has the recommended specs)
 
It's running! I had to set the settings down to 800x600 4:3. It runs smooth in the first area. I'll report back how it runs when I have more time to play it.

One thing I've noticed is I want to play it in one of the 16:10 aspects which is native, but all the lower ones run in a letter box and it's squished. all the 16:9 are also squished.

The standard 4:3 formats are not squished and appear normal?
 
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