A 4850 performs similar to a GTS250 which is a 9800GTX+. While the 4850 would provide better performance i dont think that it would be worth the upgrade. A 4870/5770 or GTX260 would provide you a decent benefit though.
The hierarchy chart puts a 9800GT equal to a 4830, and two tiers below a 4850. You wont see a very significant difference unless its about 3 tiers or more away on the hierarchy chart so i would definitely save up a bit more and get a 5770 when they drop in price a little bit.http://www.tomshardware.com/reviews/best-gaming-graphics-card,2544-7.html