How close can you go to the wattage on your PSU?

I want to get this cheap GPU (ASUS Radeon R9 280X) off trademe (new zealand craigslist). I also have a GTX 1070 which i use to mine. i have a 600w PSU. What i wanting to know is: 1.Does it matter if you exceed to the wattage on the PSU? 2. What happens if you just come close. does it do anything i should know about?