Just make sure you buy a quality brand PSU.I've seen lots of people with 700-800watt PSU's that make a bad purchase and it turns out that the PSU is only capable of delivering 500 watts.
Do you know which one it is? If it's a quality brand you might not even need to upgrade it unless you really wanted to.Although of course you would have to upgrade it anyway if you decided to crossfire or SLI down the road.