Does atx12v psu fit in all atx cases?

It should, Yes. That's one of the points of the ATX standard.

In reality, there may be a few configurations that don't quite mesh, but as a general rule, yes.

Its worth pointing out that while this is true of ATX, its is NOT true of smaller form factors like MATX. Those cases seem to use Power Supplies in all shapes and sizes.

Mostly. You may have some problems with the longer, more powerful PSU's, say over 800 watts or so.

My 550 and 750 watt Corsairs are very close to the same size.

That an excellent example of some that won't fit in a few cases. Some of those units that top 1000 watts are GINORMOUS

hey thanks all for the quick reply.