The 3870 is more in line with the 9600GT. The DDR4 will give it a lil boost and if the 9800GT is one of the low power cards at 550mhz then they will be even closer but still the 9800GT should be at least a bit better.
3870 ~ 88/9800GT. I think its going to depend on the clocks. If the GDDR4 version has higher core clocks, or you are looking at a green GT version that the 3870 will be faster. Another thing to note is that the 3870 doesn't handle AA well at all. It doesn't matter what card/clocks you're looking at. Once you turn AA on the GT will be faster.