You
can either link up the cameras before going to the graphics card, using
some
sort of switching unit, or look at getting a graphics card that supports
mulitple camera inputs (CPC sell one for around
£120+vat)
The
main disadvantage that I see with the switching method, is that the picture
would switch every xSeconds, which means that the picture is constantly
changing. (and you could miss what it was you were trying to look
at).
You
could also look at multiplexing the picture so that you had say 4 cameras
displayed on one feed.
Daniel