It is a shame I didn't made a picture from the 'update' I made in my Commodore 64 (many, many moons ago) although the lack of access to a photo-camera at that time could have contributed to that sad fact. Anyway, involved were three customized ROM's, a special designed circuit board, a lot(!!) of wires, 18 external switches to select the software inside the ROM's and a cooling fan...any combination you could make with these switches resulted in a new software goodie to play with.
That C64 really had everything you could wish for at that time...SPEEDDOS, DOLPHINDOS and even it's own graphical user interface (very similar in look and feel to the Commodore Amiga's Workbench) completely controllable with a mouse.
With the money I made selling that C64 I was able to by my first (3rd hand) Amiga 1000.

Man, happy days that were.

Later on, I repeated that feat somewhat by building an Amiga 1200 into a high tower PC case. Ah well, what can I say...the thrill was gone. Not for the Amiga (never!!!), but the custom building.
Now I just build my own PC's.