@ Foody
Uh, Foody? Your facts sound like a 1993 rumor column. There is no way to add AGA to an A500 (DCE did a prototype solution, but it was never released and never will be). And Eyetech hasn't sold much of anything in quite a while. Their website is completely out of date and their stock has supposedly been sold off to other dealers.
@ Herbman
Unless you get really, really, really lucky, the only A500 accelerators you're going to come across will be secondhand (ebay, comp.sys.amiga.marketplace). A 68010 chip is a quick, cheap drop-in solution, but its performance advantages are negligible. WHDLoad can take advantage of its Vector Base Register however.
68040 accelerators for the A500 do exist, but I've never seen one and have heard about someone having one only once.