From the Anandtech 4850 review: 
http://www.anandtech.com/video/showdoc.aspx?i=3338&p=13
Below is the performance difference between the GTX260 and HD4850 @1920x1200 resolution:
Crysis 1920x1200 :
GTX260 : 28.6	
+19.1%
HD 4850: 24
Call of Duty 1920x1200:
GTX260 : 74.3	
+11.8%
HD 4850: 66.5
Quake Wars 1920x1200:
GTX260 : 84.2	
+11.6%
HD 4850: 75.4
Assassins Creed 1920x1200:
GTX260 : 50.2	
+7.7%
HD 4850: 46.6
Oblivion 1920x1200:
GTX260 : 43	
+23.5%
HD 4850: 34.8
The Witcher 1920x1200:
GTX260 : 46.1	
+33.6%
HD 4850: 34.5
Bioshock 1920x1200:
GTX260 : 50.4	
-10.3%
HD 4850: 55.6
-------------------
Average gain over 4850:	+13.8%
HD4850 performance is not bad for a card that costs half the price ($200 vs. $400). 
And if we take into account the fact that HD 4870 will be at least 20% faster than the 4850, we can assume it will beat the GTX260 in most cases. It's good for us because this will only mean a price war, and both the GTX260 and HD 4870 should be great cards 

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