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.
i have never used the low power version of the 9800GT, I assume its similar to a 9600GT.
but I have owned or do own the following Nvidia cards.
9600GT factory OC
the GTS 250 gives similar performance to the ATI 4850 and I would not recommended upgrading from a 9800GT to a GTS 250. I suggest the next higher card, GTX 260 or ATI 4870. The performance increase you get from going one tier to the next is not that substantial in real game play.