Since your card is a 9800, it means you have an AGP motherboard so the great 8800GT won't help as it works on PCI-Ex.
The best card for you depends on your budget. Have a look at
Tom's hardware recommendation.
I have a Leadtek 7800 GS (overclocked from 375/1200 to 500/1400) that scores a fine 6700 points in 3D Mark 2005 and 1470 in the Shader Model 3 test in 3D Mark 2006 (you can see
here how it stands, e.g. it beats the 1950Pro
which is considered the best AGP card).