Generally speaking if a power supply is advertised as having mutiple +12 volt rails you can add the individual rails to get a rough approximation of the total current (amps). The total will not be exact. It will usually be a little less. High quality power supplies will come very close to the combined total. Low quality psu's will have a lower total.
The vast majority of power supplies advertised as having mutiple +12 volt rails do not have true separate rails. They have only one large rail. Think of them as having "virtual rails". The advertising gurus really stretched the truth. Big stink over that a few years ago. There are a few power supplies that really do have separate rails. They are usually high end, high wattage models
For the typical user and the typical gamer single and multiple rails are not an issue.
Here is a link to an outstanding article by jonnyguru that explains multiple rails:
Usually, the manufacturer of the GPU recomend the PSU with a system configuration like this:
Core 2 Quad extreme to full load, Velociraptor, GPU to full load, RAM to full load....
Some moths ago, I install my X4 955, a GTX260, 2x500GB Itachi, DVD-RW,CD-RW, 4GB Crucial Ballistix Tracer and a case computer with 3 fans....all this with a 500W PSU Thermaltake, so, your current PSU is more that enough for the system.