I think the XFX Pro 850W Core Edition would be a good option, especially if it is from a trusted seller and if you have ambitions to upgrade to two high end cards in the future.
According to these reviews:
http://www.hardwaresecrets.com/article/XFX-PRO-850-W-Po...
http://www.jonnyguru.com/modules.php?name=NDReviews&op=...
The efficiency is highest between 20 and 50/60% load, which is about 400W.
The Thermaltake I recommended should be available at other etailers, such as Scan.
£65
http://www.scan.co.uk/products/675w-psu-thermaltake-tpx...
Antec Truepower New TPN-650 80Plus Bronze Modular £65
http://www.scan.co.uk/products/650w-psu-antec-truepower...
Yes you are correct that just because a unit is 80Plus certified does not mean that it is high quality. And that is not to talk about how some brands don't legitimately use the 80Plus symbols.
Efficiency is just one aspect of the performance of a PSU. There are other aspects such as voltage regulation - where the voltage on each rail (3.3V, 5V, 12V etc) has to remain within 5% of the nominal value. Ripple and noise suppression, where ripple on each line must be controlled (50mv and under on the 3.3V and 5V rail, 120mV and under on the 12V). Then there is build quality. It's fine having good performance as long as the PSU lasts at least as long as your actual system does. But there won't be any performance at all if the PSU is not design to run in the conditions in which it is kept (temperature rating) or if it fails after a few months etc. Just because a PSU has some level of 80Plus certification does not mean than any of these other aspects of performance are up to scratch.
Those are more or less the essential ones. Then there are more optional ones such as the size of the fan and the speed at which it runs. The number and length of the connectors. The size of the case - some large PSUs are incompatible with smaller mATX cases.