Around 300W.
However ATX sized I don't know many units in that range that are actually decent.
Super Flower Golden Green HX comes with a 350W version
Seasonic has a couple of units like the G 360W, S12II-350 & the Platinum & X-Series with 400W.
But those are usually the same price or even more expensive than decent 450-550W PSUs like the XFX TS/Core/Pro 450 or the Corsair CXM450/550