'Should' is the operative term. It SHOULD work so long as the rest of the computer isn't taking up too much power. We can theorize about it using PSU calculators and reviews for hours. If you're having doubts or plan to upgrade your PC anymore you probably could do with a bigger power supply. Note, the operative term is 'probably'. Every system is different.
Yes, with that kind of amperage on the 12V rail I agree, it SHOULD work. I hope that gives you more confidence in making a decision
Personally, I would choose a 650W PSU just to be safe. Then again, I like to play it safe with a large safety cushion. That, and I want a PSU that will last through several builds with increasingly higher demands. Nvidia suggests a minimum of 500W for the 260 and 550W for the 280. It's up to you where you want to settle.
Calculating total amperage is an extra step. Amperage is a function of voltage and wattage. The wattage is constant, but the voltage is not. The rails in the PSU have different voltages for various parts of the computer. There's a 3.3V, 5V, and 12V with a -12V, and 5VSB (stand-by, I'm guessing?). With constant voltage, watts = volts*amps
In most power supplies, a majority of the amperage is on the 12V rail. My 650W PSU has 52A on the 12V rail. 12V*52A = 624W max on the 12V rail at standard testing conditions. Your video cards run on the 12V rail, so when looking at PSU's don't look at their overall rating as much as the amps on the 12V rail.
For example, my corsair 400cx has 30A on the 12V rail (360W). It replaced a Viotek 450W that had 22A on the 12V rail (264A). Even though it is rated higher in wattage, the amperage could not power my video card. The corsair unit is also $50 while the Viotek unit is $20. The corsair unit will last for years on an aged PC with end-of-life products; the viotek unit IS an end-of-life product that is arguably better than a doorstop. If you're looking for quality PSUs, look for good brand names. This is an integral part of your 'homework' before buying a PSU.