I'm pretty sure performance will scale the same both cards. The HD4870 is faster in both games in DX10 and it should be faster in DX9 as well.
Here are FC2 numbers, DX9 vs. DX10 with the HD4870 and GTX280. In conclusion, DX10 actually performs better, although they do use Vista for both tests.
There are some older tests with
Crysis, but updated drivers have probably altered the performance.