You can set one aside and get one part to match the other.
For the 20 pin ATX board, you would need an ATX power supply like the M2ATX. The PS you have is more of a 12VDC regulator, which works with computers that require a 12Vdc output "power brick". I see that in your searching that may have been hard to get. I think (but have no first hand knowlege that) the dell sx270 needed 12 vdc power input, and had it's own internal power supply that further distributed from there.