I second the recommendation for the ACA500+ it is an amazing device that speeds up the A500 pretty well on its own but can use A1200 accelerators too. I also like the built-in Action Replay, RAM, CF media cards/HD and ability to boot different kisckstart versions on the fly as well as easily switch between PAL and NTSC.
The only downside is that it is plugged into the expansion on the side. So if you are only interested in internal expansion, then it might not be ideal.