Hey. First of all, wow, a girl that appreciates the fact her boyfriend likes computers, and wants to buy something for it *thumbs up*
You are correct in thinking that they are the same card. They are just manufacturered by different resellers of the same product, by ATI.
The 4850 I'm sure would make a fantastic choice for him, and should last him a good while, as it is a pretty new card, and a good one at that
In terms of the reseller you get it from, Sapphire isn't a bad choice at all.
Something to know, is that the resellers don't really usually change much about the card itself, if its from ATI or Nvidia.
They may just change the sticker/fan, add a game as a freebie, and of course offer different warranties, depending on how good the reseller is (if that is the right word).
Some do change parts of the card, to help performance, but not many.
Sapphire though is known, and I have used one of their cards previously with no problems.