Did a bit of searching on the internet, the best mods, in my opinion seem to be these two:

Widescreen Fix
Arc Trooper Conversion Mod

Anyone have experience with these or have any other mods or texture packs they have used before?

Haven't played this game before but want to get the best possible experience before I start.

Edit your system.ini
Star Wars - Republic Commando\GameData\Save
find following
(the numbers will be whatever there - game doesn't give you ability in game to set higher than 1600's)
below is for 1080p, you can set whatever resolution you want.
FullscreenViewportX=1920
FullscreenViewportY=1080

I wouldn't say it needs widescreen fix, or arc trooper mods.
What is nice you can set cache for engine from 32mb up to 1024mb

find this line
this is set for 1024mb your should have 32 there.
[Engine.GameEngine]
CacheSizeMegs=1024

you can also use this
UpdateDynamicLightmaps=True
but it might cause crashes as i remember.

Is Arc Trooper an add on or a complete conversion?

Complete conversion

Indeed, thanks. In case someone needs it, FullscreenViewportX and Y appear repeated a few times. The only items that is necessary to change are the ones under [WinDrv.WindowsClient]

EDIT: Repeated.