I did some research and found mixed "reviews" comparing the two. Some say it's at least 2x faster, some say not that big of a difference. Majority of the reviews say it is noticeably better than the 9800GT. It's worth the $75 as well. I have one and can run many games I have on maxed out settings (Rift, Fallout NV, ARMA2, Portal 2), there hasn't been a game yet that I haven't been able to run maxed out. I have an ATI Radeon HD 1G.
It's also difficult to compare. Some games are built for Nvidia cards such as Crysis, making a benchmark test sometimes give varying results depending on the game.
What processor do you have? I have an Intel i7 860.