How much will a 40gb hard drive for a computer hold?

Assuming the 40gb is free, available space, you could hold approximately:

10,000 High-Quality MP3 Songs

-or-

25,000 High-Resolution Digital Photographs

-or-

120 Hour-long (43 minute) television shows in high-quality.

-or-

700,000 Word Documents between 3 and 5 pages long.

Etc, etc...