Actually, Bert has the concept of efficiency and power ratings backwards. A 500W PSU
should be able to supply 500W of sustained DC load. If it has a 80% efficiency at full load, it will be drawing 625W AC.
All of this is what the power supplies should be able to handle. The sad truth is that many PSU manufacturers tend to overstate their actual power capabilities or simply don't stress test their units properly.
That all being said, you don't want to be running your PSU at it's full capacity all the time anyways. It'd be like redlining your car continuously and expecting it to work after 3 months.
The 750TX should be able to handle the two 5850's you have with some headroom to spare. I'd check the actual power draw using a
Kill-a-watt, and then estimate or look up the efficiency to determine the DC draw on the PSU. A safe estimate would be that between 30-70% load, the PSU would be 80% efficient. Otherwise, count for 75%.
The only two websites that I really trust for PSU recommendations are
SilentPCReview and
Jonny Guru. Both have the methodology to properly test power supplies to their full capability. (I'm a little biased in this regard since I used to work for SPCR).
