Apple's video card choices suck, plain and simple.

Not only are the video cards in the iMacs underpowered, they have been isolated as the main reason for the freezing issue. I think the MBP got a decent break with the 8600gt, but it is by no means a screamer. The mini has older-than-dirt integrated GMA950, and the Mac Pro's video cards are almost three years old in some cases.
The base video card in the Mac Pro (nVidia
7300GT) is
17 months old and pathetically underpowered. It's like putting a 2-cylinder engine in a Mustang.
The
x1900xt in the Mac Pro will be
three years old this coming January. Some people had to have their cards replaced due to artifacting. Apple had Foxconn produce a revision 2 of the card that supposedly cut down on the artifacting.
The
Quadro FX 4500 is about
two and a half years old (July 05) and has been superseded by several generations of professional-level graphics.
Apple prefers cards that are quiet, and in doing so have and will regularly de-tune the cards to lower clock speeds so that fan speed can be kept to a minimum. Apple never takes into account graphics performance, or anything related to gaming, because quite frankly, His Steveness doesn't want you playing games on your Mac. He wants you to make edible garbage with iMovie 08 and post it to YouTube.
Pouring salt in the wound, Apple charges
outrageous prices for these video cards, when you build-to-order. Want to add a second 7300GT to your Mac Pro?? It will cost you $150.00, when it can be purchased RETAIL for $50.00 or less. How about the x1900xt?? $400.00 for that card, where (if you can even find it anymore) it costs $150 or less. Consider this: an nVidia 8800GTS that would literally spank the x1800xt costs about $250.00. So for an extra $150.00 you can have a three year old card.
They could have done
so much better on the video card options in the iMac, but they wanted high margins and low noise. And they could have at least updated the Mac Pro's at least ONCE since its inception a
year and three months ago. The Mac Pro has been virtually
untouched since its inception.
Apple has
no grasp of video card technology, given their long history of poorly-performing and absurdly expensive graphics card choices.