Yami, thank you very much for the reply.
I know about the form factors -- but I did not think of this in regard of building a new PC.
(I can replace a HD or a DVD-drive, but I have never built an entire PC. I remember how I swore and cursed when trying to get the HP motherboard out of its original SFF-case: I swore never to do such a thing ever again, haha. HP changed the little connector pins on the motherboard: it is not possible to connect the RESET key of this XGear case. OEM: gaaaahhh).
When it is time for a new computer (I have already been doing some reading), the content of the PC now will (probably) discarded. The CPU is just a Pentium 4 of 1.6 GHz. The motherboard, an Asus P4B-MX, only has an AGP-slot (no POI-e), 3 PCI slots and and IDE-connectors (no S-ATA). Yes, old stuff
This machine is not my main computer (of course). But I plan to keep it as long as it works fine.