well, the resolution difference is quite important. You can calculate the number of pixels each GPU has to render.

So, equal graphic settings (except resolution).

GTX 480 has 1920 x 1080 = 2073600 pixels = 30.5 fps

HD 6870 has 1920 x 1200 = 2304000 pixels = 36.6 fps

the resolution difference equals to 11.11% more pixels rendered for the 6870.

The 1 Ghz helps it a bit, but Unigine is not really CPU hungry, especially since it's built to show what tesselation is all about.

Anyway, the performance difference looks solid.