I 'upgraded' to a new ATI graphics card as well and could not play Dom2 anymore! I had to wait for a couple of month until ATI provided a newer driver, which supported Hardware-OpenGL and now it runs fast...
Although, I am referring to running on Linux above, I tried it on Windows once, too. The results are the same: The native Windows does a bad job, but the drivers that you can download from ATI's website worked well enough for me.
Otherwise, quite a while ago on this board, someone recommended the "Omega"-drivers. They worked with my old ATI card well enough, but for my new one the drivers from ATI's website were good enough as I said. Try google on "ATI Omega Drivers"...