OS X uses the graphics processor to accelerate a number of system tasks, and the GUI in general, like Windows 7, benefits from being able to offload graphical processing. For $100, on a system that you can't upgrade, I'd go for the better graphics chip.
A Mac with 4 GB of RAM, a Core iX CPU and a decent, if unspectacular GPU is going to be good for many years yet, that $100 will be nothing amortized over 4-5 years.
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-Scott
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