You should be fine for now. I have seen these run on 300W power supplies without a hitch. I would recommend the upgrade though, as soon as you can, just to know that your in the clear. I have been running an HD7770 on 350W PSU in my sons machine for over a year now.
Don't know how much your planning to upgrade but get a PSU with some PCIe power rails so when the time comes, you will be set for the extra power needed for newer cards.