You can only use 1 mod at a time, but here are some brief discriptions of each of the ones I know enough about:
1. Ply's Mod
Enables dual wielding and adds many extra scripting commands. Contains Airmod features.
2. EfxMod
Provides incredible graphical enhancements. Contains Airmod features.
3.Airmod(Not reccomended because Ply's and Efx both have it)
Enables down the sights aiming if you have animations set up.
I'm not an expert, so I probably missed some stuff, but that's what is most important to me about each of the mods.
Must... help... bunny... achieve... world... domination... all... hail... bunny...