My laptop only has 128M of memory, so I'd consider running a lightweight database like DBD::SQLite instead of something like MySQL or PostgreSQL to reduce memory requirements.
FWIW, I don't think the amount of memory you have would preclude you from running either PostgreSQL or MySQL. I have a fairly old laptop P150/80MB and it runs mod_perl, PostgreSQL, X and a browser pretty well. Not great, but certainly sufficient for coffee-shop development and demo purposes.