The A500+ normally has a 2 MB Agnus inside, with 1 MB of ram on the motherboard, all of it chipram. To get 2 MB of chipram, you would need a trapdoor memory card with 1 MB. 2MB is the maximum amount of chipram you can get on an A500+.
The BBOAH can give you some extra info on the (relatively minor) differences between a normal A500 and an A500+.
