I believe that you have two PSUs, two CPUs (1300 and 1800 Durons), two mobos, (both K7SOM non +) and at least one stick of memory, but I'm unclear as to what that stick is.
So here is what I want you to do...
Remove the power supply(s) and the motherboard(s) from the case(s)
Set the whole lot on your Dining room table.
Just one stick of memory in the slot closest to the CPU, with the CPU and it's HS/F properly installed.ABSOLUTELY NOTHING ELSE
Connect just the ATX lead from a PSU, a 'case' speaker, the keyboard, and your monitor. ABSOLUTELY NOTHING ELSE
Plug in the PSU, and momentarily tap the 'pwr on' pins with a screwdriver.
Do you get any 'beeps', or anything up on screen?
With it laying on the table, and it still does not come up, then try the other power supply.
Then try different ram, (either SDRAM or DDR), if you can.
And a different CPU, if you can.
Repeat the above sequence with the other mobo.
Are you certain that the heat sink is squarely on the CPU?
With the motherboard out on the table, it is a lot easier to bend down and look at it from the side, to ensure that the heat sink is making proper contact with the CPU core.
It's away-too-easy for anyone to put the HS/F on backwards (and I STILL do it!
, and then it doesn't make proper contact.
And, if it doesn't, the CPU will very rapidly overheat, (like in seconds, not minutes), and you can totally fry your CPU.
eddaweaver is partly correct... all recent PSUs are ATX, and are interchangeable, but cases and mobos can be either ATX or micro ATX.
The full size ATX mobos you are considering will not work well in a micro ATX case.
Easy way to tell... 2 CD-ROM bays = micro ATX
Three (or more) CD-ROM bays = full ATX
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