How can I estimate the maximum load on shelves on a plasterboard wall?
Your wall probably is not just plasterboard.  Its much more likely that your wall has 2x4 studs (vertical boards) behind the plasterboard spaced 16" apart.

Knocking, by the way, is a remarkably unreliable method for finding studs. :)

The load bearing capacity of drywall/plaster/etc should be treated as practically nothing.  What you should do is find your studs - buy a stud-finder OR drill holes in a straight line horizontally across your wall spaced 1" apart, until you find one that is harder to drill into than the others.  Then measure 16" in both directions horizontally from that hole and drill again - and so forth to find your studs.

Once you know where the studs are, mount your shelving brackets into the studs using 3" wood screws (Pre-drilled holes please!).  

Anchoring into the wood properly will ensure that your shelves will stand up to pretty much anything you'd normally see on shelves.