If you are looking at the GTX760 then your power supply is more than enough. Even then you could probably push it a little more (R9 280X works just fine on 550Watt too) although I am not sure about the R9 290/x that some people have mentioned previously.
Although I will say in regards to your worries about heating issues that is only a problem with the reference cards. Anything non-reference like the Asus DirectCU2, Gigabyte Windforce, MSI, etc will run fine.