Wattage isn't the only thing you should be looking at. You should be looking at reliably built PSU's. Pertaining to this thread's topic, I'd pick a 650W reliably built unit for headroom and with the intention of holding onto the PSU for a little while. I'd pick a 750W reliably built unit if I want to hold onto the unit for a long while and also perform overclocks on the platform the PSU and GPU will be paired with.
What is the make and model of your unit and how old is it?