I am very new to Illustrator and have version 8.0. I know nothing about
version 7 and don't know if this will help. This is the first thing I
learned in Illustrator since I have a need to reproduce a nib pen type of
line, thick and thin as you say.
I drew a long,tapered outline using the pen tool. Filled with black. Then I
selected the path and dragged it into the brushes palette. I filled out the
proper boxes in the dialogue box - most importantly: proportional. After
that I applied the brush stroke to paths. It works very well and looks much
like a hand -drawn line from a nib pen.
I learned this from the March issue of MacWorld, page 98.