I made the following chart for PCs.
It may be applicable to Macs with Windows.
Or it may be of interest to a Mac person with extra space and money who wants to buy a PC rig for gaming.

My objective was to find the most powerful video cards for running X-Plane.
X-Plane is the only game I play.
I scanned the tech/gaming sites for benchmarks.
X-Plane is OpenGL.
I could only find one big name game that is OpenGL and is being used for benchtests; Enemy Territory; Quake Wars.
I took 35 benchtests of tech/gaming web sites using ET:QW to get my results.
I started out with a benchtest from ComputerPowerUser which had 6 video cards.
That benchtest was very close to the average results I was seeing in the other benchtests.
I then compared card to card.
For example, I calculated that the GTX 285 was on the average 4.8 fps faster than the GTX 280.
I would plug that into my chart and I would adjust it if necessary.
I am confident that the first 10 video cards are pretty much in the correct order because they are very consistent with the results of most benchtests.
One caveat: the GTX 295 was inconsistent. Four web sites believe the GTX 295 driver has to be improved.
Xbitlabs said that the GTX 295 failed when they enabled extremely high levels of antialiasing.
I only used single video cards for my results, no sli or cf.
The HD 4890 overclocks well and will rival the GTX 285 when OC'd.
As you can see "bandwidth" is not necessarily indicative of the results.

For Macs it is a lot easier. The HD 4870 is the best card followed by the HD 3870 and the 8800 GT comes in third place.
I am still talking about X-Plane. For other games the 8800 GT will probably beat the HD 3870.
My hope is the HD 4890 will become available for Macs.
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Do not argue with an idiot. He will drag you down to his level and beat you with experience.