Mostly core speed and memory speed. A retail 8500 has speeds of 275/275. An OEM 8500 BBA (built by ATI) is 275/250. A retail 8500LE is 250/250. An OEM 8500LE BBA is 250/230. Ati Partner cards (you'll notice the "Powered by ATI" markings) may and do have different specs.
Performancewise, at the same speeds or overclocked to the same speeds, any two ATI 8500 variants (with the same amount of memory) should perform just about the same.
The retail ATI cards have the Rage Theater chip which offere some improved video handling but this does not affect gaming performance.
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