That depends on the PSU. You're looking at 400W+ overclocked. But I wouldn't do that with a low quality 550W PSU. Get a good quality unit if you want to overclock.
I might would consider 650W since you have the budget. It's always good to have extra power you can use if you want to ever upgrade your GPU. Also, 650W or even 750W would handle 400W better than a 550W unit.