I would not want to use that PSU for a high-performance gaming system, but you have what you have.
http://www.jonnyguru.com/modules.php?name=NDReviews&op=Story6&reid=341
Power-wise, 400W of 12V power (about 33A) is plenty for a GTX 960. The 380 uses 70 to 80W more than the 960, so I would not want to run it on only 400W unless it was a very good PSU. (It isn't) Also, since it has two 17A rails, a 380 might peak at more than a single rail can handle and cause a shut-down,