The A1200 motherboard is in the 41x19cm range and the old PS3 is 33x28cm~ish so you wont fit it inside
The A600 motherboard will fit tho
Fitting the Motherboard would not be to hard, take any PC distances and mount the motherboard, a dremel will make good use for making a hole for the floppy drive.
You should print out a cutout for the ports on the back and dremel them also, check voltage of the PS3's leds and if needed use resistors and you can use them as H/F-disk and power leds.
There is lots to do but manageable, getting the touch buttons to work will take some work, a slim slot-in DVD-drive would be manageable to fit nicely but not all drives will work since not all drives allows use of slave drive!
You will also need a interface to connect a PC keyboard tot he Amiga.
But go for it if you feel up for it
