I'm after the above, really.

I need to be able to draw out what the database will look like and how it'll work, and be able to export this into some portable universally readable format (pdf, jpg etc.), and ideally also export the SQL script to create the tables.

I've been playing with this but it's a little tedious and the only way I can think of to export into a form I can show consists of sticking together multiple screenshots.

Any suggestions?

Ideally something rather lightweight, as I'm relegated to a PIII box at the minute...

I haven't tried doing exactly what you are, but one of these programs might do what you are looking for.

Tried this ?

http://fabforce.net/dbdesigner4/