I'm also going to build a system soon, to play CloD - so would also be interested to hear feedback from owners of HD 69xx cards. Currently I've settled on the 6950, rather than a 2GB Nvidia GTX 560, but that could all change...
Although the data sample is still small, the results on my site
here show that systems with the HD 69xx cards seem to be performing well - with good average FPS on the Black Death track.