- what is an appropriate (simple) amplifier for a small speaker (or is
      the 5 volt the limit, and should I use either a higher voltage or a
      4 transistor bridge)?

Why do you think you need anything more than a single transistor to Ground?

    - would a piezo buzzer make more noise (when driven with the same voltage)?

Yes, put you'd have much less flexibility in the type of sound.

    - does anyone have experience in generating more interesting
      sounds via 1 output bit (for instance using PWM)?

There exists IBM PC (original) software that can use the internal speaker
(which has a single bit, single transistor to ground) for multi-tone music,
synthesized voice, and all sorts of stuff.  My understanding is that it
works by combining a PWM signal with a high frequency signal designed to
"bias" the speaker cone near the middle of its travel points (in combination
with physical inertia.)

BillW