The small circuitboard in the trapdoor is most likely a memoryexpansioncard. Most trapdoor RAM-expansions were 512 KB, effectively doubling the amount of ram your A500 has. Many of them also hold a realtime clock (including battery).
If you just want to play games: stay with what you got, although an extra (external) diskdrive is very nice to have. Not only for playing games, but also for making the highly important backups of the disks you have.
You don't need extra ram, new roms, a harddisk or whatever if your just playing games. Al those things _can_ make life easier, but most games will not make any use of them.
Things to consider:
- make sure what type of Amiga you have. It appears you have an A500, but make sure it is;
- chances are the ramcard in the trapdoorexpansion has a battery backedup clock. Chances are the battery is leaking and thus damaging the circuitboard. Check the battery for leaking and remove it if it does;
- get an extra (external) diskdrive.
