Usually the total is less. Looking closely at the label on the side of the PSU will tell you. Usually there's a second line below the output currents telling you what the maximum power the 12 volt line will provide. It's usually in watts, so divide by 12 to get amps.
Generally on the smaller (sub kilowatt) PSU's, they are only nominally separate rails. So do not worry about which rail feeds what components.
In addition to what jsc wrote, some psu's also have limiters on the 12v rails which effectively limit the amps to the label rating. Higher end psu's (Corsair, PCP&C, etc) do not have limiters on multiple rails and can provide the total amps on one rail as noted on the second line as mentioned by jsc.
Generally speaking, one of the components that you should not cheap out on is the psu. I'll never understand how someone can spend $800 on a mobo, proc, and memory and then buy a $30 psu. I highly recommend the Corsair 750TX or 850TX psu's; quiet, realiable, and stable with plenty of amps on the 12v rail(s) for whatever you throw into your machine.
chunkymonster - For a typical system with is usually just one video video card a 750 or 850 watt power supply is overkill.
Tell that to the folks who purchase the PCP&C 1000W psu and only have one gpu. And perhaps it is, but given the price/performance of a 750W psu and given that some (most) folks use the same psu build after build, it is in my opinion worth spending the extra $ not having to worry about whether that $50 450W psu can handle additional hard drives, overclocking, and/or a second gpu. Heck, I just retired a 660W Enermax, at the time of purchase it was a little pricey, but after 5 years and 3 builds later it was money well spent. I also firmly believe that the psu is one component of any build that you should not cheap out on and the extra amps and watts really don't add anything to the monthly electric bill.
I would also say he's wrong. Limiters do just that. If the total 12V output is 34A, then it is limited into two 17A rails. Most PSUs have OCP (Over Current Protection) which is what limits the virtual rails to 17A. This can be set incorrectly, or not at all to allow full output on a single rail. PCP&C is known for not using multi rail setups, they should show only one rail on their systems. I've read reviews on jonnyguru where Corsair/OCZ has OCP enabled, but set at really high values. It seems like only Antec/Seasonic use OCP "correctly".