There isn't just a mod for beautiful graphics in GTA:SA, there are hundreds of mods.
Before you mod your GTA:SA, please remember that if any mod gives you unfair advantage over any other player, it will get you banned.
Anyway, you'd probably start by installing ASI Loader, what does it do? It basically allows the usage of .asi files in your GTA:SA. Here's a link to download:
http://www.gtagarage.com/mods/show.php?id=21709Simply extract the vorbis dlls to your GTA San Andreas folder and you're done.
Then I would recommend getting modloader, what does it do? It simply allows you to drag and drop mods into a folder and they'll appear in-game, magic. That means you won't have to keeps hundreds of gta3.img backups and stuff.
For texture overhauls, there's a few I know of:
Project RoSA - It's basically the HD version of GTA Textures, no true changes.
HRT 1.3 - Total texture overhaul, trees, roads, everything.
Project Oblivion - It makes trees totally realistic, I think it's incompatitble with a few things though.
There are hundreds more, I would try gtagarage.com, gtaforums.com and moddb, they have countless mods.
Then you have ENB, I know little of that, but I'm sure someone else could explain it. You need a really beefy PC for that though.