There was a prototype of an AGA upgrade for the A500 by Phase 5 (who else?) that was shown at Amiga shows but never actually was released due to Phase 5 going out of business. It had the AGA chips and a 68020 or 68030 and Simms on a PCB. There are photos on the web somewhere!
Edit: it was from DCE and has PPC in addition to AGA. A photo is in the amiga.org gallery
http://www.amiga.org/gallery/index.php?n=1242
