For videocards, there is no difference with the exception of the Radeon 8500.
Oem's have no box and usually come with just the drivers on a CD. Most of the time they'll have all the cables, dongles, included, but sometimes these are missing as well.
The OEM Radeon 8500 is clocked lower than the retail version.
I've also heard rumors of OEM cards using slower RAM, but this is mostly limited to the Radeon 8500 LE.
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