load the whole database into RAM

hello,

I use pgsql to store data for my program (hotel manager). The
filesystem reports that the size on disk of database is not so bing
(~100MB  ~20MB actual data). I want to tune the engine to fit the
database as much as possible in RAM, say: postgres, please, use 512MB
(1GB) of my RAM just for your needs and     load everithing on it. Is
it possible and, of course, effective?

Just taking a look to some tuning document I didn't find a direct
answer (I'm not a smart guy)

Thanks,

Filippo