If you want this calculation done automatically (without using VB), there a little bit more to it...
Create a design table in the model including the dimensions required for this calculation. Then add an additional column labeled $PRP@Volume with the appropriate calculation underneath.
You can add the note then as Scott stated above or insert the string $PRP:"Volume" directly into your note.
In several cases, I have also linked the dimensions from Sheet 1 of the design table to Sheet 2 (and more) for more complex calculations and then linked these results back to additional custom columns on the Sheet 1.
It makes for a very simple down presentation method to show how minor design changes affect the end product because the notes can be included in your model views in addition to you drawings.
One additional thing, if you use the additional sheets in your design table, be sure to go back to the Sheet 1 (you cannot rename this sheet also) before closing or SW will error out.
Good Luck....
Remember...
"If you don't use your head,
your going to have to use your feet."