I always shied away from OEM versions for the possibility they might be different enough to cause problems. I've had experience with OEM versions of ATI cards and found them incompatible with others... even other ATI's. So, my rule has always been "No OEM or other 3rd-party cards" unless it's the only one in the machine.
I asked Matrox tech support about this years ago, and they said the Matrox and OEM branded cards were "identical". But I know that was not the case for ATI's.