Grand Thieft Auto San Andreas is the most epic game in series by amount of gameplay features. Quality difference is much bigger than between GTA Vice City and GTA 3, so modding of graphics have reason. First version of ENBSeries was developed for this game 3 december 2007 and had SSAO with SSIL only. Game is very popular for modding and even now many peoples playing multiplayer version of it (SA-MP). I like GTA Vice City much more, because it's more "romantic" and atmospheric for me, but GTA San Andreas is rude.
Different versions have own visual look, latest have not finished reflective bump (dudv environment bump mapping) for rainy weather.
Some mods working improperly with SAMP, because several functions are hooked by SAMP and not return proper result to ENBSeries. I'm developing absolutely new version, but it's long process and i don't have much time for it.
You may see comparison screenshots from this mod and vanilla game in the gallery or on the gtaforums.com. Of course everything can be modified by yourself.