In addition to the many changes you can make within the NFSHP2 Options screens, you can also make some important changes through editing the .ini files spread throughout the various directories of the game. You can use any text editor (such as Windows Notepad) to edit these files and make the appropriate changes. Before you make any changes though, make sure you make a backup of each file you're going to change (copy it and rename the original) so that if things go wrong or you want to go back to the original settings, you won't be stuck.
Below are the main tweaks I currently know of, and more will follow as I update this guide. I don't cover settings which can be changed in-game, as it's recommended that you make such changes there and not manually in the .ini files.
This file can be typically found under your My Documents\EA Games\Need For Speed Hot Pursuit 2 directory.
Setting this to 0 will mean that NFSHP2 will start in windowed mode. This is generally not recommended and you should run the game in fullscreen mode.
Setting this to 0 will allow you to choose resolutions above 1280x1024 in the Screen Resolution setting under the Options>Customize Graphics screen within the game menu.
The rest of the settings in this file are best changed in-game (such as bit depth and resolution).
This file can be typically found under your \NFSHP2\frontend\gui\system directory.
This is the screen resolution at which NFSHP2 starts up and displays the menus. The default is 800x600, but if you've set your resolution higher (or lower), when the game goes to load up a race the display switches resolutions. If this annoys you, you can change this setting to match your "racing" resolution, for example change the 800,600 above to 1280,1024. However, the menu will still display at 800x600 so unfortunately if you raise this resolution you will see a smaller menu screen displayed (as it maintains its 800x600 size). As I said, changing this would mainly be to eliminate annoying screen resolution switches.
This file can be typically found under your \NFSHP2\EAGL\ directory and controls the level of detail (LOD) for graphics. You usually adjust these settings using the in-game detail sliders on the Customize Graphics screen. However, if you want to experiment with certain effects, edit the various settings in this file for the level of detail you've chosen in-game. Instructions on what each setting does are at the end of the .ini file.
There are many other .ini files throughout the NFSHP2 directories which can be edited to alter vehicle dynamics, special effects, track listings, etc. I would strongly recommend that you do not alter these without using the editor, or without knowledge of what you're doing. The simple tweaks provided above and the in-game settings should be more than enough to allow you to customize the major game settings. And of course, as more tweaks come to hand I'll update this guide accordingly.