If I were you, I'd go with the retail. You don't wanna miss out on all the extra g00dies and stuff that come with it. However, if you were buying, say, a cpu, definitely go with oem.. there isn't really anything extra that comes with the retail besides the warranty.
I'll tell you what the difference is, cause I bought a OEM, your speed on the OEM will be 300/530, and the retail will be 325/600?, get the retail its faster and you can overclock it easier.
Most definitely. And you could probably overclock it well beyond the retail card's standard specs. Of course, it will only overclock as well as the retail card, if as good.