A single 5870 can do 4.2 MD5 ghash/sec in whitepixel. A 5970 probably like 7 (in an x4 rig, they got 28.6 Ghash/sec).
The assumption was a 100k rounds of SHA-512, not a single round of MD5. I don't know if the 10k trials / sec is correct, I just assumed that the OP had it right. It translates to roughly 1 gigahash/second of SHA-512 per 5970 which sounds slightly high to me but is probably ok given the assumption that GPUs wll probably keep getting more powerful each year.