Yes, please get at least a 550W PSU: http://www.guru3d.com/articles_pages/radeon_r7_260x_r9_270x_280x_review_benchmarks,10.html
I know your components are different than guru3d's but he takes the "average" system into consideration and your system is right there. Not a place to mess around. At best you may get away with it working for a while but after time you'll likely experience random shutdowns, crashes, hangs etc and worst case wear down your psu for no further startup if you don't upgrade this 400W to at least a 550W psu as recommended. Hope this helps!