-you say your proc is 1.6 ghz, but is it a P2 or P3, or P4, or AMD?
-What is your mobo's buss speed?
Anyhoo, displacement would be my first guess as to why it may be taking to long.
Also, do you have a screenshot you could show us consering your displaced object(s).
More than one displaced object obviously will add to the calculation time too.
..sry man, just shootin in the dark here
Israel "Izzy" Long
Motion and Title Design for Broadcast-Film-DS