Like enzo said for that CPU and onboard graphics a 300 watt PSU would be still more then you need to run your system for just about any single GPU on the market will run with 550 watt PSU there are a couple that need more but for the most part 550 watt will give you plenty of power to run a single GPU system. Now I cant stress this enough do not get cheap with the PSU its is about the most important component on your rig so get a good quality unit its not always about the most wattage its more about the quality of the unit.
Also check out my little Graphics Card Power Requirements guide
700w would be plenty, but cpu isnt going to need it all, depends on what gpu you want to run really. im running radeon 5770, i7 920 and asus p6t se on a ocz mod xstream and have never had any problems
That could run on a good 400 watt psu.