Get at least a 1200 or 850w power supply. They last a long time and you can use the same one when upgrading parts over time in your machine.
It would suck to get a 600w power supply, then later down the road realize you need a 800+ one to run the machine when you want to upgrade. This is a common mistake every newby builder makes.
It's a good investment to spend the extra cash on a bigger Watt power supply. Otherwise you're just gonna be spending more money down the road when you want to upgrade.
Your advice is dangerous. A 450W PSU, like Rosewill Capstone, is enough to run any single card configuration and then some, with room to OC very very much.
A 1200W PSU can power like 4 single card systems running at full gaming load.
OP has a 600W Corsair, it is top of the line and will last forever. 2 GPU's just fine. Dont recommend sticks of dynamite.