> iPad5 points like a dual G5/1.8ghz

Yes, minator already mentioned this in another thread.

> where do I plugin various SATA HDs?

I don't know about "various", but SATA isn't necessarily a problem any more:

https://morph.zone/modules/newbb_plus/viewtopic.php?forum=3&topic_id=7675&start=272

That was 4 months ago, so there's probably even more by now.

> can I fit a modern PCIe gfx card?

While there're various ARMv7 SoCs with integrated PCIe controller, I've yet to come across a board with ARMv7 SoC and PCIe slot.

> can I have an ATX board that beats or even emulates last QuadG5?

To quote myself from this thread:

"Today's fastest ARM CPUs are still slower than the PPC970 CPUs in most (i.e. 2.0+ GHz) G5 desktop machines."

> it should be easy to find a dozen of boards that can be
> bought in almost all european countries, right?

Right. You could try to start there:

http://en.wikipedia.org/wiki/List_of_single-board_computers#ARM_based