Turns out I do have a faulty HD3870 card, waiting for a replacement so got a HD3870x2 loaner which runs OK on the P5W DH Dlx, seems to lose out very very slightly to 2 x HD3870 though.

HD3870x2 Score: 16089
http://service.futuremark.com/compare?3dm06=4919830

2 x HD3870 Score: 16142
http://service.futuremark.com/compare?3dm06=4918330