Who was the original composer (probably 19th century, like Debussy or 
something) of the melody that is sampled in the song Paparazzi by the group 
Xhibit?

Xzibit sampled Maurice Ravel's "Pavane" in the song Paparazzi. Maurice Ravel 
was a composer in the late 19th/early 20th century (1875-1937).

For more information read:
Review of hip-hop, 3rd paragraph about Xzibit
http://www.ctheory.net/text_file.asp?pick=225
Maurice Ravel
http://www.hnh.com/composer/ravel.htm

great.

Actually, var-ga is close, but not entirely accurate.

The song is called Pavane, and it was by Faure (who was in many ways a
mentor to Ravel).

Ravel's work is called "Pavane for a Dead Princess", Faure's is simply
called "Pavane".

Mister X-to-the-Z's production team chose Faure's piece, and I don't
think they could have picked a finer track.

More: Fauré's Pavane (op.50) is not a song, but a piece scored for a
small orchestra. Later on choral parts were added using a poem by de
Montesquiou.

Excellent reference on Fauré here:
http://w3.rz-berlin.mpg.de/cmp/faure.html