The 9800GTX is an 8800GTS (G92). So think of 9800GTX+ as being an overclocked 8800GTS.
The 4850 should be the better card.
also you don't have to go through the rebate proccess the 4850 is better choice
Those two cards pretty much trade blows with the 4850 being a little faster as Turtle1 said. My only grip with the 4850 is that most models are single slot solution which is not the case with the 9800 GTX+. This helps keep things a little cooler. IF you're willing to spend a little more some, you should get a none reference 4850 equipped with a better cooler. I know HIS has few models that seem interesting. The 4850 has a tendency to run hot so a better cooling system is certainly a good idea beside the fact that the heat get expelled from case.
the 4850 is better at Higher Resolutions as well, just an FYI...