That is a fairly low power system, do you plan to overclock? If you dont want to overclock and dont intend to get a higher powered GPU in the same system later down the road i would recommend a quality 400-500W PSU, you could even get away with a 400CX corsair with only that 5770 and its usually around $30 on sale after rebate, cant beat it for the money.
If you want to overclock that i7 or just want some more breathing room for a future SLI/Crossfire setup or just a power hungry single card(GTX480) I would go with a quality 650W or maybe 750W unit.