The Chips. OCS were the earliest. This version of Agnus allowed only 512K of true chip ram. If there was a 512K trapdoor board, it was neither true chip ram nor fast ram. Soon, the next version of Agnus (don't know the part number, but I think most A500s had this chip. All 4 of mine do.) began appearing. It could address 1 meg of true chip ram, but a modification was required on the mother board to reconfigure the trapdoor ram as true chip. With the A3000 the last ECS Agnus went into production, it could address 2 megs of chip ram. The A600 and A500+ also used this chip. All 3 Agnus versions had the same pin-outs AFAIK. I think that both 1 & 2 meg versions were called ECS chips. There was also an ECS Denise chip that added a few seldom used (by me anyway) screen modes. I don't think these made it into too many A500s. The new screen modes required ECS and OS 2.x or better to access them.
If you've got the 1 meg Agnus, the rest is easy. The Denise chip, if you can find it, probably isn't worth the trouble. You can do the MB modification (procedure on Aminet) if someone hasn't already done it yet. You can update the rom to taste or go with a rom switcher. Fast Ram is a great idea. You don't hear much about it now with hard drives in good supply, but, some people with lots of ram used to configure a Rad: drive (a second floppy helps with this). This was one of the coolest things an Amiga could do (I just love this machine). Grin.
The possibilties are endless. It's easy to find a hard drive, interface and fast ram that go on the side, a CD rom and other SCSI devices can usually plug into the back of the interface. If you want to play a vintage game without the expansion, some have defeat switches, and even if it doesn't, you can simply detach it.
If you want acceleration, you'll have to choose your expansion path with more care, so all the new components play well together. You just have to decide what you want to do with the thing.
