I'm using the coolermaster extreme power series and it is perfectly stable. It is perfectly capable of running the 260 sonic, and now the 275.
In theory at least, I doubt an 80+ 460W can give the same amount of power as a 70+600W. I don't know about the amperage though. Will have to check.
Either way, for a g card, it is the 12V rail that matters, and if it can adequately supply the juice to run it, then there is no problem whatsoever.
Listen, you're a good poster but you need to know a few things first.
1. Efficiency of a PSU 80%(80+) does not mean it can at maximum deliver 80% of is rated max output(i.e 80% of 460=368w). It means that at max load or at an average(different ways to measure) the PSU outputs 80% of the power that it draws from the wall, i.e why a 450w rated Corsair PSU can provide upto 570w. Follow-
So 80% efficiency: say your system draws 250w from the PSU, so its drawing(250/8 )*100=312.5w from wall.
70% Efficiency: 250w draw, PSU draws 357.1w from wall.
SO higher the efficiency lower the relative power consumption, means your electricity bill stays down.
2. The max output of a PSU HAS NOTHING TO DO WITH EFFICIENCY, it has everything to do with the rating and quality of the internal components. Corsair and seasonic(also tagan, pc power & cooling and lot more) uses high quality components and under rates the SMPS while cooler master does not do that for extreme series PSU, so while a VX450w can deliver 570w(not to mention more clear SNR ratio, i.e clean power) the CM Extreme power 600w may only deliver about 550-600w max, and also the poor build quality of CM extreme series makes it vulnerable to over voltage related damage, about a dozen of my friends use all sort of PSUs from CM to Corsair and CM has the highest failure rate among imported smps(CM extreme-2 failures, Corsair-0 failure, zebronics, frontech and etc local made- about 12 failures so far).
I don't doubt that CM600w can power GTX260(maybe GTX275 too) but be assured that the lifetime of the PSU will be lower and the failure rate higher that any corsair or tagan smps.
God, I can't believe that I wrote something this long.