You'll have to disconnect all the cables going to the motherboard on top, and remove the power switch, which pops out the front. Remove the screw from the centre of the motherboard (which you would have removed when you removed the plastic motherboard holder), and then slide the motherboard towards the front of the machine, and remove.

There's probably a couple of things I've forgotten, since thats for an 8100, but it should still be pretty much the same.