personally i think that around 1200-1500 triangles for a fighter should be fair enough, and shouldn't hit so much the performance in slower machines, like mine, as soon as you do all the lods, and maybe if you increase their number too: in this way the higher poly LOD will appear only when a fighter is very close.
more again, i doubt that an higher polycount will give you higher visual quality, since you will not notice on a small model like a fighter all the small details, expecially during action
personally i think that once you reach a good enough level of details, what can increase more the quality are textures instead of more details