How to enhance graphics and/or enable wide(er)screen for SC1
Ok, so there is a lot of scattered information across these forums and other places on the interwebs, so in an attempt to consolidate all of this information, I am posting it here in this guide into 10 easy steps. Most of it is copied and pasted. I'm not trying to take credit away from anyone, just trying to put it all in one spot because I spent at least 2 hours scouring/trying to understand everything. So here is the quick and dirty way to do this. Please note however, you may want to tweak some of the below settings to your own liking.

Step 1. Go to the installation directory for Splinter Cell. (D:\Program Files\Steam\steamapps\common\Splinter Cell\system)
Step 2. Open SplinterCell.ini.
Step 3. Replace everything under [D3DDrv.D3DRenderDevice] with this:

Translucency=True
VolumetricLighting=True
ShinySurfaces=True
HighDetailActors=True
UsePrecaching=True
UseMipmapping=True
UseTrilinear=True
UseMultitexture=True
UsePageFlipping=True
UseFullscreen=False
UseGammaCorrection=True
DetailTextures=True
UseTripleBuffering=True
UsePrecache=True
Use32BitTextures=True
HardwareSkinning=False
AdapterNumber=-1
ReduceMouseLag=False
UseTripleBuffering=True
; ForceShadowMode flag : 0==projector, 1==buffer, unspecified==auto-detect
ForceShadowMode=1
EmulateGF2Mode=1
FullScreenVideo=True
UseVsync=True

Step 4. Save.
Step 5. Close the .ini and then right click on it, go to properties, put a check in "Read-only", and click "OK".
Step 6. Open SplinterCellUser.ini.
Step 7. Replace everything under [Engine.EPCGameOptions] with this:

AnimatedGeometry=True
ShadowLevel=3
LightMapsLevel=3
TextureLevel=3
Resolution=1920x1080
AmbiantVolume=99
VoicesVolume=99
MusicVolume=99
AudioVirt=0
Sound3DAcc=false
EAX=false
InvertMouse=False
FireEquipGun=False
MouseSensitivity=50
ShadowResolution=3
EnableFadeOut=0
FadeOutNearPlane=1000
FadeOutFarPlane=1250
SFXVolume=99
Brightness=50
Gamma=50
EffectsQuality=3
FadeOut=0
VidMem=2
GraphicsCaps=2
EAX_Capable=False
AntiAliasing=3

(Comment: For Resolution=, enter your monitor's actual resolution or your desired resolution. In this example, my resolution is 1920x1080 so that is what I entered)

Step 8. Set DesiredFOV and Default FOV under [Engine.PlayerController] = to one of the following based on your monitor's ratio:

4:3 --- 75
15:9 --- 87.61661884
16:10 --- 85.2812718522
16:9 --- 91.3154482287

For me personally I have:
DesiredFOV=91.3154482287
DefaultFOV=91.3154482287

For Reference:
15:9 Resolutions
1280x768

16:10 Resolutions
1280x800
1440x900
1680x1050
1920x1200

16:9 Resolutions
1280x720
1920x1080

Step 9. Repeat steps 4 and 5 for SplinterCellUser.ini.

Step 10. Open nVidia control panel (or go to similar location for ATI cards) and enable FXAA either Globally or just for Splinter Cell.

One down side to changing the FOV is that the mouse/cursor becomes misaligned. I have not found any way to fix this. For example, with the keypads that you come across throughout the game, button coverage with your cursor is as follows:

100% - Normal coverage. The entire button has mouse-click detection.
75% - Normal mouse-click detection except for the right quarter of the button.
50% - The left half of the button is selectable.
0% - There is no mouse-click detection on the button. You must click the left area beside the button.

First column
4:3 - 100%
16:9 - 0%
16:10 - 50%

Second column
4:3 - 100%
16:9 - 50%
16:10 - 75%

Third column
4:3 - 100%
16:9 - 100%
16:10 - 100%

This guide has been created from the post by 'TheJudge25' that can be found here. I am posting this as I have been unable to get him to make it a guide and wish to do so for my further personal use and others.