If you want to stress test some kind of worst case scenario, you can go singleplayer and put 30AI and yourself, use mods are they are usually less optimized. If you have no stutter solid 90fps on first lap then you are good to go with whatever settings... Multiplayer should be less taxing on CPU anyway.
Then you can play with your graphics settings.
As for Graphics just check the menu it is pretty quick to go around everything.
Dynamic reflection 8 is good but you can go higher...
FSAA 4X or 8X
AA on mirrors
Texture Filter 16X...