Also can improve with console commands (dont know if they all work now, coz I used them many years ago when had bad PC). Before that copy and safe your Config.wtf somewhere.
/console ffxGlow 0
/console ffxDeath 0
/console ffxSpecial 0
/console set shadowLOD 0
/console fixedfunction 1
You could set game to 16 bit:
/console set gxColorBits "16"
/console set gxDepthBits "16"
And set your max FPS to 60 (human eye cant see more anyway). You could do that in game options or /console set maxFPS "60"
After all console commands I think you needed to restart the graphics engine to enable them with:
/console gxRestart
There was more configuration oprions with console, but its all that I can remember for now.
P.S.
There where lots of guides and solutions for that on blizz forum, so better try to look there.