Hi,
Look up the charts on Toms. There is a hierarchy there, which shows roughly which models from ATI and Nvidia are similar. In some games the 4850 will beat the 9800gtx/+. In others the 9800gtx will edge out the 4850. They are very similar performance wise. Either will do well up to about 1440 x 900 with high/full settings in today's games, and naturally a semi decent rig too!!. Alot of the time the difference between the two, is which games is configured better for which card ( "Nvidia the way it's meant to be played"!!)
Personally, I have a 9800gtx, Q6600 @3.0ghz, 4gb Ram. I've never been happier gaming wise. It eats everything i throw at it. Sometimes more than I expected. Either one is a good purchase.
Hope that helps.