If you're using stock voltage the TDP shouldn't increase much. At stock settings, 5850s are only ~130W cards so without increasing voltage, I doubt they'd go past 150W. 150Wx2/12V= 25A. Depending on your CPU, it probably has a TDP of only up to 150W tops, so add that in and you're looking at around 37.5A max draw. Add a little extra for peripherals, HDDs, etc and I would have a hard time believing you could get it over 45A.
The only real concern is whether the PSU is of good quality. Some PSUs will advertise false or skewed numbers that are actually only achievable under very specific conditions, like only 1 rail being tested at a time and not the 3.3 or 5.5 rails either. Other times they flat out lie and in product reviews, they find they can't draw anywhere near their max without shutting down or blowing up. I don't know much about HIPER so I can't really say whether it's good or not.