Is this psu ok?

Solution
"400w is a too low a wattage for any serious build."

Most gaming builds use ~250w while gaming, so realistically a good quality PSU that is able to output 400w is fine. Though this unit isn't.

It provides 336w on the 12V rail at best probably (it may not even be able to provide that much under normal conditions).

As mentioned, you need to provide more detail for a better answer. What system? Are you buying this PSU? What retailers and budget?
Could you provide a bit more meat to the feast ? What kind of build are you planning on using this ? What motherboard and components have you chosen ? 400w is a too low a wattage for any serious build.
 
"400w is a too low a wattage for any serious build."

Most gaming builds use ~250w while gaming, so realistically a good quality PSU that is able to output 400w is fine. Though this unit isn't.

It provides 336w on the 12V rail at best probably (it may not even be able to provide that much under normal conditions).

As mentioned, you need to provide more detail for a better answer. What system? Are you buying this PSU? What retailers and budget?
 
Solution
I would ask the OP to provide Make and Model of Computer/motherboard, cpu, video card used. to suggest a proper psu.

The circle 400w is described in theses words on amazon, and by the cables for system components it describes this is an older system psu and should not be used on any newer system, as it doesn't even have 6+2 video card cables, and is only 77% efficient, making is a poor mans psu indeed.

FEATURES • ATX 12 V Version 2.31 supports dual 12V Rail • 400 Watt power supply unit for system with high configuration • Energy saving design with high efficiency more than 77% • OCP, OLP, OPP, OVP, SCP, UVP Protection • Intel, AMD. & also supports Dual Core CPU • Super silent 12cm sleeve bearing thermally controlled fan • 20 + 4 pins (Nylon sleeve), P4 + P4 (Nylon sleeve) , 2PATA (Nylon sleeve), • 1 FDD, 3SATA (Nylon sleeve