"A single HD 4870 scores 1783ppd while using Stanford's Folding@home GPU client and drew 349W while folding. Using a single instance of the folding client with the HD 4870 X2, only generated 1,237ppd while the system drew 334W. Clearly both cores weren't engaged and we couldn't, at the time of writing, convince an instance to run on each of the card's GPUs."
From this review
of the HD 4870X2