No, you can't just add the two amperage numbers together. Those are the max amparage for each rail, not the max for both rails together. To find that, you divide the 300W total for both 12V rails by 12. 300/12=25. This means that both 12V COMBINED can put out 25A. This should be enough to power a 7600GT.
Thanks for the reply, so your saying I dont need 20amps on a single +12v rail to run the card? Which does sound wrong considering even to the best SLI cooler master PSU's costing $250 only have 18amps on each of the +12v rails.
Pretty much everything runs off the 12V. Every molex plug has a 12V and 5V wire in the plug. (5V is red, 12V is yellow/orange.) Your CPU also uses the 12V rail. The 4pin square plug that is now needed is what supplies your CPU with the extra 12V power that it needs. If you were to send 12V to all devices in your computer that needs it with only one wire, you'd have a very dirty 12V line, a fire hazard, and a lot of issues. PSU manufacturers spit the wires up so that the load is distributed more evenly, and you don't overload the small wires.
Short answer: no, you don't need all 20A on one line because you need to consider the computer as a whole. Most devices need 12V power, so as long as there is enough to go around, your good.