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I am by far no video card genius, but from what little I do know, most video card makers plan on making it backwards compatable to some degree. They don't want to limit the market to only the computers coming out now, instead they want to make it possible for the others with AGP slots to buy one also. Some features probably won't work as fast, but you do have to sacrifice something on slower systems.
 
Well the GF3 wasn't backward compatible with AGP 2x. Nor was the GF2 I believe. At least now those older AGP Powermacs can enjoy the graphics we do by only paying very small price!
 
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