For builds in this budget I advise hopping on craiglist.
Flagship fermi and vliw4 GPUs like the HD6950 and GTX580 go on craigslist for as low as ~$50 these day. These cards at that sort of price will give you more visual quality in modern games than spending more than twice as much on a new GPU. If the budget has room, ~$100 on CL will often buy middle-tier GPUs that are actually still architecturally current, but are just behind by a model generation. As an example, I've bought HD7870XT's (2 of them) for $100 each on CL, and that was like 4 months ago. These are very similar to the render performance of an R9 280.
900P and 1050P monitors go for $25-50. (I've bought nice DELL brand heavy/sturdy LCDs, 1050P, for $25 each).
2x4GB DDR3 will often go for ~$40 or less (I've paid as little as $25). I recently bought a 2x8GB kit of Crucial memory that works just fine for $60 on CL.
For the CPU/MOBO, if you have a microcenter within 50 miles it may be worth the drive. They almost always bungle an FX-6300+GA-78LMT-USB3 for ~$100 (pick up only).
I've found 120GB SSDs on CL for ~$40. (brand new in package, original receipt included!).
Buy a NEW PSU of good quality no matter what you build.
$400 can make an awesome gaming computer if you're willing to do some leg work and take on a little risk.