Delete your GTA San Andreas User Files folder, or change it's name.
Make a shortcut to your gta_sa.exe
Right click the shortcut and choose properties
You'll see a field named Target, containing the path of the .exe e.g.
"C:\Users\Zeus\Downloads\GTASA\gta_sa.exe"
Change it to:
"C:\Users\Zeus\Downloads\GTASA\gta_sa.exe" -width 1024 - height 768
Click OK
Double Click the shortcut to start SP
Set your favourite (supported) resolution inside the game menu. I strongly recommend you to use something > 800x600 as your current display driver does not seem to support it anymore.
Close SP
Start SA-MP
done.