What "games" do you want to play on a $400 rig? MAME and 8-bit 16-bit emulated games? Most "gaming" rigs I build $400 is just the cost of the 2 video cards...
I could prob throw something together for twice that, but for 400, something has to give. Probably more than one thing has to give; You wont be able to use latest hardware, and also wont be able to order good quality hardware. For 400, you are buying the cheapest hardware you can find(which is not recommended) and still wont have something powerful enough to play the new Far Cry 3, for example
In other words, you could build one, but it wont last long, or play many games