I have searched the internet high and low to find the best of the best FAQ's and walkthroughs and here is what I have found so far

http://www.geocities.com/pdranking

This site explains exactly how the ranking system works with an in depth text file. TOP SITE! I highly recommend u check it out. This where I got most of my information from its an awesome site.

http://db.gamefaqs.com/console/n64/file/perfect_dark_combat_simulator_a.txt

This text file explains everything u could imagine about the combat simulator it rates maps , tells you how to beat all the challenges and much more.

http://db.gamefaqs.com/console/n64/file/perfect_dark_combat_simulator.txt

This text file has strategies for every level in the game, in depth guide on all of the simulants, tells you how many challenges you need to beat unlock levels/weapons and much, much more.

 http://db.gamefaqs.com/console/n64/file/perfect_dark_challenges.txt

An in depth analysis of challenges and other cool stuff

http://www.gamefaqs.com/console/n64/code/25092.html

CHEATS, CHEATS, CHEATS !! There are some kewl cheats on this site

http://codes.ign.com/codes/8/3906.html

This is a great site for cheats

http://guides.ign.com/guides/3906/

This contains everything for perfect dark the only catch is u need to be online

http://www.gamesdomain.co.uk/perfect_dark_combat_simulator_a_gd.txt

This is a huge text file that contains loads of information about perfect dark stuff 

http://cheatplanet.com/n64cheats/perfect_dark.htm

This has everything u could ever want to know about perfect dark

http://cheatplanet.com/n64cheats/strategy/perfect_dark.txt

This has some stuff on all aspects of the game