First, load up the game and see if your desired resolution is selectable. If it's there, set it. Now quit the game and go into <gameDir>\System. If you were unable to set the custom resolution, keep reading. If you could, skip down to the part that says **FOV INFORMATION BEGINS HERE**

Go into <gameDir>\System. Open up the .ini file that has the name of your game. Deus Ex's file is deusex.ini, Unreal Tournament 2003's file is ut2003.ini, and so on. Find these lines (which may or may not be near eachother):

FullscreenViewportX=
FullscreenViewportY=

Change the values on the right to set your custom resolution.

**FOV INFORMATION BEGINS HERE**

Now open the file user.ini. Find these lines (which may or may not be near eachother):

DesiredFOV=
DefaultFOV=

Change these values to set the custom FOV.

Now for some lists:

Games for which this is confirmed a full solution:
Brothers in Arms: Road to Hill 30
Clive Barker's Undying
Deus Ex
Land of the Dead: Road to Fiddler's Green
Harry Potter & the Chamber of Secrets
Tribes: Vengeance
Unreal II
Unreal Tournament
Unreal Tournament 2003 (for 16:10 displays)
Unreal Tournament 2004


Games for which this is confirmed a non-solution:
Advent Rising
America's Army
Deus Ex: Invisible War
Harry Potter & the Prisoner of Azkaban
Harry Potter & the Sorceror's Stone
Pariah
Splinter Cell
Splinter Cell: Chaos Theory
Star Wars: Republic Commando
SWAT 4
Thief 3: Deadly Shadows
Unreal
Unreal Tournament 2003 (for 16:9 displays)
The Wheel of Time
XIII