HOWTO: getting black borders on a widescreen display

[Jonathan Brandmeyer <jbrandmeyer@earthlink.net>]

I've been using Debian for several years, and am currently running Sid.
I recently installed Starcraft under Wine, which insists upon running in
640x480.  My monitor is a 20" widescreen (1650x1080).  The modes picked
by the NVIDIA driver all stretch the screen to the full width of the
display, but I would prefer to maintain the actual aspect ratio of the
pixels and display black borders on either side (or just one side) of
the screen.

Any ideas?
Thanks,
-Jonathan