Because power supply efficiency forms a kind of a curve when measured they tend to be more efficient at about 20%-80% load so it's actually recommended to have some extra power draw available, but not too much.
I have an 250-300watts max pc running with a G-550 seasonic which runs at maybe 90% efficiency or more. Hardwaresecrets review loaded it up with 221 watts dc power with 246 watts ac power consumed.
http://www.hardwaresecrets.com/article/Seasonic-G-550-Power-Supply-Review/1683/8
If it's going to run at 350 watts at max with synthetic benchmarks and tests or some other extremely stressful application, I would say a 450-550 watts would be more appropriate.
If it's running at 350 watts while running a game or some other...