https://en.wikipedia.org/wiki/Grand_Theft_Auto%3A_San_Andreas 
http://www.rockstargames.com/sanandreas/

The original version 1.0 is most compatible with mods and generally recommended. There are two (identical) executables "for compatibility reasons", "the Steam release of San Andreas changed the EXE name - so Steam looks for gta-sa.exe when it attempts to launch San Andreas. Mods on the other hand look for gta_sa.exe as this was the original retail EXE name." 
http://gtaforums.com/topic/753764-steam-downgrader/

You can find fixes and improvements for the game here