Or would I be better off with 600w?
a minimal of 500 watts is the recommended amount of watts for the R9 270x so yes it is enough
This is very similar to the build I have, and my biggest regret is getting a PSU that just big enough but not for upgrades. I do recommenced getting a 550+.
but if you want something with better upgrade paths yet similar perfromance then I would suggest this: http://uk.pcpartpicker.com/user/ironviking/saved/dB3H99
I generally don't recommend PSUs under 550W for gaming builds. You don't need more than 650W, even for most dual video card builds (some need more), but it's nice to have the option of expandability.
But if your budget is tight and you don't think you'll run more than 1 GPU, then 500W will be just fine.
500w will be fine, but for the sake of possible upgrades or overclocking, etc you might want to snag a 600w.