I thought OEM was somethign like, "Out of the Box Experience" or something? Oh well, OEM is just retail without the fancy boxes. I bought an OEM video card, it was just a brown box with the video card in it and some cables.
Bascially, if you want the simplest version of a product, get OEM, if you want all the upselling things, like, say you purcahsed a 6800GT, in the box it would say like, "Buy a new Sound Blaster blah blah blah." That's the only difference...