The rail overload would only be typical on PSUs with two or maybe three +12V rails, since the video cards would be forced to share power with peripherals. PSUs with four or more +12V rails will not exhibit the problem since the video cards already have their own rails. Also, some good-quality PSUs have regulators to regulate the voltages across all the rails, which helps avoid an overload.
As for the "isolated power" comment, I just reread it and it suddenly sounded like a bunch of crap. I read that comment on the PC Power & Cooing website and just stated it here without actually thinking properly about what it meant. Some electrical engineer I am, eh?
Isolated power is only possible in superconductor materials. There is minimal wastage of power with multi-rail PSUs, since there are rarely PSUs that have two or more truly separate rails fed by separate transformers. Usually, most PSUs have a single transformer that generates a certain number of amps. This is then broken up between two rails, and these rails are rated by how much current each rail can carry independently. Marketing creates confusion in that when you have 30A being produced by the transformer and being fed to two +12V rails rated at 17A each, most people will think that this means that 34A is being produced. This is not the case, since while one rail may be loaded to 17A, the other can only carry 13A since that is the maximum power being generated by the transformer. This is why you cannot add PSU rails together and is the reason most good PSU manufacturers supply a maximum wattage rating for the +12V rails combined.
Fact is, a good-quality multi-rail PSU can be more energy-efficient than a single rail one, due to the fact that a multi-rail PSU has a secondary transformer coil with extra MOSFET switches. So as long as the MOSFETs used for each rail are of high-quality (>95% efficient, so as to give at least 85% idle power efficiency) and you don't load the extra rails, the switches stay closed. So while a single-rail PSU with a high-quality MOSFET may start with an idle efficiency of 85%, a multi-rail PSU starts with each rail efficiency at 95%.