OK, I'm going from memory here, but I do believe that most mounting systems intended to go on an AR put the center of the optic about 1.5" above the rail (pretty much where the sights are with a fixed carry handle). Whether you will be able to see the front sight in the scope depends on how powerful the magnification of the scope is, or more specifically, the field of view in terms of degrees.
In simpler terms, the lower the magnification, the more probable it is that you'll be able to see the front sight. For example, with a red dot that has almost no magnification and a very wide field of view, you can definitely see the front sight. if I remember right, you can usually see the front sight with scopes up to about 4 power. Generally speaking.
Of course, this is just what I remember. I don't run any sort of irons any longer and don't have any to check out.
In any case, if the front sight gives you problems, you do have alternatives. The least destructive would be to replace your front sight based with a railed gas block or something of the sort; or, if you decide that the rifle will become a dedicated optics rifle, you can just take a dremel or hacksaw to the front sight base and take the top of it off. I'd go with option 1. If you cut the FSB, there's pretty much no going back.
Edit: more info came in while I was typing.
