for the difference in price between them i'd just opt for the 4850.
same on the nvidia side. a gts250 isnt that much more expensive than a 9600gso
worst case senario you get to turn on a few more options in the game.
if its just for video playback then go for the cheaper card.
"Im building a gaming PC"
He still should post his resolution as there will most likely be better cost vs performance options....or if his budget supports it he should opt to get a nice new 22" monitor....