Is it possible to use an ASUS P6T motherboard in a Dell Dimension 9100 case?
If I remember correctly, The Dell uses a BTX form factor motherboard and the Asus is an ATX form factor. The BTX is slightly bigger (325x266) as opposed to (305X204) for the ATX, so in theory it should fit, however, it will also depend upon the motherboard stand-off alignment and the backplate.
I remember trying to do a similar thing a while ago (different dell and a gigabyte mobo) and it didn't really gel.
Best advice is to take the existing mobo out of the dell and position the Asus over the general area and see if the holes line up and see if there's room for the Asus backplate.
You will probably have to consider the power supply, also...