The 4850 beats the 9800GTX+ in almost every bench so I am sure the 4870 will utterly smoke it and for $60 more that is a great deal.
Nvidia cards at this time are not a good price vs Performance buy.
The only thing that brings them close together is the added cost of an aftermarket cooler, but for me it doesn't matter what card I buy, I always pull the factory cooler off, put it in the original box with all the other stuff taht came with it (in case I need to RMA it) and put a good cooler on.
What I am hoping for is that my Thermalright HR03 R600 cooler from my 2900XT will fit so I won't have to buy a new one...
Here's a couple of charts from Toms hardware...
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"Every normal man must be tempted at times to spit upon his hands, hoist the black flag, and begin slitting throats." -Henry Louis Mencken
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VEGETARIAN -Noun (vej-i-tair-ee-uhn):
Ancient tribal slang for the village idiot who can't hunt, fish or ride.