What will your eyes see?
Radak: The destruction [of Edom].
Malbim: The vengeance that Hashem will take against Edom - "v'Shilumas Resha'im Tir'eh" (Tehilim 91:8).
Why will you say "Yigdal Hashem me'Al li'Gvul Yisrael"?
Rashi: His grandeur will be seen above our border, to inform that we are His nation.
Rashi citing Targum Yonasan: Hashem's honor will increase, and He will widen the border of Yisrael.
Radak: You will praise that you dwell in your border, and their border is destroyed. Your border will be called Gevul Yisrael, and theirs will be called Gevul Rish'ah.
Malbim: Just like evil will be lowered, and its border will be desolate, so Hashem will increase His honor above Gevul Yisrael - when this one (Yisrael) rises, this one (Edom) falls (and vice-versa) - "Imal'ah ha'Charavah" (Yechezkel 26:2).
