I have some A500 mobos. Two of them are plus.
I have also some Ram cards for the trapdoor. One of them is 512KB chip so my A500+ has now 1.5MB Chip. I don't know if your chips are compatible but if u can do the hack so u could add fast ram later through your trapdoor.
3 days ago I bough some chips for my A600 to add 2MB chip :-)
Goodluck mate.