Overall 4850 is equal to 9800gtx+ so I don't see how can a 9800gt ultimate
(128 sp and lower clocks than 9800gtx+) be the better card, especially not with AA turned on,which i think that most of the people who by cards in that price range ,use it, plus hd4850 has much more potential and is more future proof than the 9800gt ultimate,even the 9800gtx+. But ,yes, 9800gt ultimate can sometimes
outperform hd4850,but also 8800gtx,8800gts 512 or even 8800gt can outperform hd4850 in some games,but that doesn't make them better cards.
If you can find 9800gt ultimate cheaper than hd4850 for ,like15-20$,then go for it,otherwise,go for hd4850.