at this website you can calculate the power supply you need:http://extreme.outervision.com/psucalculatorlite.jsp
I've done it already and just by the information that you gave me, you'll need on average about a 900W power supply, so power isn't your problem! (unless the power supply is faulty and not giving even power distributions throughout your circuit!
have you installed the latest drivers, supporting the gtx 400 series and sli capabilities?
have a professional check your power supply for faults, or if you can do it yourself?
the things is older power supplies never really had anything to work around until the 5970 and 480 came out, and now that they have to work a bit, they tend to get a bit faulty1 happened to me and a power supply upgrade fixed it all...
not to be funny or anything but I never heard the name of topower? is it a good brand? remember a brand name says it all, and sometimes we only pay for the brand name, so if you are in the market to purchase a new PSU, lean towards good brand names, because you don't want them to mess around with those great GPU's of yours
Just my thought