In terms of aligned marks - being the marks that you're awarded by the Board - no courses are capped (as Minai pointed out).
However, a maximum scaled mark is imposed on some courses during the scaling process when UAIs are calculated. You can read about how this is done in the
Report on the Scaling of the 2002 NSW HSC (produced by the Technical Committee on Scaling and published by the UAC).
Under this 'capping of scaled marks' regime, English (Standard) and English (Advanced) are
never capped.
In addition to this, only the top few percent of students are affected. The caps are not unreasonable - certainly not as low as 80%! The vast majority of courses have a maximum scaled mark near 50 (on a 1-unit basis).
I've constructed a page displaying which courses have, in the past, had a maximum scaled mark imposed on them. You should pay more attention to the 2002 listing than the listing for 2001, as it was produced from the same formula that will be applied for your HSC.
You can access it here: <a href=/capping.php target=_blank>Maximum Scaled Marks</a>