The GX260 came in three different packages -- Small Form-Factor (SFF), Small Desktop, and Small Mini-Tower. It was the Small Desktop package that used the PCI riser, not the Small Form-Factor.
Yes, but when you remove the power supply from the SFF, you expose the second PCI slot, which is normally hidden under the custom power supply.
I guess that allowed Dell to use the same mobo for both the SFF and small desktop unit.