Keep in mind that modern graphics cards (even entry-level) can easily support upwards of a million polys onscreen, with the high-end ones handling over thirty million at 60fps. Most of the stress in modern games is from the enormous shader load associated with everything onscreen, for literally every single lighting calculation and special effect (both textures and geometry) that exists in the game.
I'd worry not so much about the unit's polycount (1-7k is perfectly fine), but rather the types of textures you put on it and the shaders that are applied to that texture, and other effects the unit has.
Keep in mind that modern graphics cards (even entry-level) can easily support upwards of a million polys onscreen, with the high-end ones handling over thirty million at 60fps. Most of the stress in modern games is from the enormous shader load associated with everything onscreen, for literally every single lighting calculation and special effect (both textures and geometry) that exists in the game.
I'd worry not so much about the unit's polycount (1-7k is perfectly fine), but rather the types of textures you put on it and the shaders that are applied to that texture, and other effects the unit has.