First I'd like to thank you guys for supplying all the information about the case here. It was a tremendous help.
I got an empty G5 case (empty as in without logic board, hard drive etc., all the fans, cables etc were still there) from a friend...for a couple of beers. I rebuilt the case to fit a microATX board. It was actually pretty easy. I'm not much of a craftsman, but I think the result is pretty nice.
Front (not much to see here):

Back:

This photo is without hardware, because I'm still waiting for my new power supply and two 92mm fans for the back to arrive. I was unable to get the original Apple back fans to run, so I need to replace them with standard PC fans. I am going to use the original fan mount though.
The LED, USB connector and power button work. I don't have a FW connector on the main board, so I didn't bother to rewire that. I also don't need the front audio connector, so I didn't rewire that one either.
This is what I did:
I first removed everything in the case, including all the fans and the metal sheet that separated the drive section from the logic board section and the drive holder. Then I removed the original distance pieces (very easy, just tilt them with pliers and they come right off). Then I roughened up the ground of the distance pieces, screwed them to the main board and applied glue (you know, the type of super glue that is supposed to glue a building to a running train) at the bottom of each piece. Then I literally glued the main board right on the correct spot. Saved me a lot of measuring. When the glue dried, I unscrewed the main board again.
Obviously, the cut-outs for the backside connectors are incompatible with ATX main boards. So I had to cut a hole here. I wanted to keep the original fan grill though, so I had to move that one too. Eventually I removed a big part of the back and replaced it with a large aluminium plate (2mm strength). For the main board connectors I bent a 0.8mm aluminium plate into the form of a small tray, cut out the spaces for the connectors and attached it below the upper plate. Then I cut out two large circles in the plate and attached the fan grill to it. Finally I bolted it all together on case. Eventually I also painted the whole thing to make it look more like coming from one source. Unfortunately the paint I chose darkened when it dried and it doesn't quite fit the color of the rest of the case. I also had to use a brush (I don't have an airbrush) and looks a bit amateurish if you ask me.










Anyway, let me know what you think and I hope this description helps someone when rebuilding their case like your posts helped me. Thanks again.