I think the ATI 8500 was the last time there was an actual performance difference between a retail and OEM card. Only when a card is OCed by the manufacturer or if you plan on OCing and one card has a better copper HS would it really make a difference. However, if a retail version has a game you want included for a price difference lesser then the game itself, then that is a factor as well.
For the X1900XT line up at this time, there is no difference, even in the cooling. Just what games come bundled and maybe a VGA to DVI adapter which you should not be using anyways.