For A500+ : Mini Mega Chip 2.1 OR MegAchp 2000/500

[Matt_H]

Expanding on zipper's comment, you need something like this that slots into the trapdoor. There are lots of boards like this out there that can be bought from your preferred vendor or eBay.

The MegAChip and such are for the regular A500 (non-plus), which is only capable of addressing 1MB of Chip RAM (512K onboard and 512K in the trapdoor). The MegAChip makes those machines capable of addressing 2MB of Chip RAM: new Agnus and 1MB RAM on the MegAChip + 512K on the motherboard + 512K in the trapdoor.

[Matt_H]

ACA500plus is a good choice, but you should still get a trapdoor RAM expansion. If you're new to the Amiga, note that it has two different kinds of memory: Chip and Fast. The ACA500plus will give you Fast RAM, but only the trapdoor bus can give you Chip RAM. A full 2MB of Chip RAM will make life much, much easier for whatever you're trying to run.

[Matt_H]

2MB is the limit for Chip RAM. Every A500+ has 1MB onboard and can take an additional 1MB via the trapdoor. Like zipper mentioned, you need a 1MB trapdoor expansion suitable for the A500+. There are lots of these on the market, vintage and new, like the one I linked (Individual Computers, maker of the ACA500plus, also has one). Just make sure it's an A500+/1MB expansion and not an A500/512K expansion. If it has a clock/battery on it it's for the plain A500 - the A500+ plus has a clock on the motherboard and so its trapdoor expansions don't include one.