Actually ATI don't build is own board. Many ATI board are made by Sapphire and sold by ATI.
I'm not sure if Sapphire is building ALL the "Built by ATI" board. Maybe other users will have more detail about this.
I would buy any Radeon based board as long as the MEMORY chips are good. Because some manufacturers use cheaper memory chip to keep there retail price low.
For exemple, my board is a "Built by ATI" Radeon 8500 128 Megs. The memory is clocked to 250 MHz be default, but the memory chips are RATED to 275 MHz. I overclocked my RAM to 300 MHz without any problem. If I would have buy a "cheap" 8500, maybe I would have get 250 MHz rated memory, so I would not have been able to overclock my memory that much.
Another thing to consider, is the cooling, some manufacturers have better cooling than other... So you can overclock these boards easily.
I hope this will help you a little bit!
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Would you buy a GPS enabled soap bar?