Between the 4850 and 8800GT its going to be about $50 extra for the 4850 for about 15-40% more performance depending on which game you're playing.
If you're just going to play Aces High, save the $50 and get the 8800 GT. If you're going to play other games that are more demanding than Aces High, I'd say the 4850 is definately worth the extra cost. IMO.