720p on a 1280x1024 PC monitor - sorry, that doesn't happen in practice. It simply doesn't happen. Why? Two reasons:
(a) PC monitors automatically scale to fill the panel (1:1 monitors are rare, especially in the common 4:3, 19'' 1280x1024 non-HD offerings).
(b) PC gamers will prefer to use 1024x768 (correct aspect ratio & full-screen) instead of 1280x720 (incorrect aspect ratio *or* black bars) on a 1280x1024 monitor. Think about it.
Again, the 720p resolution is completely irrelevant in PC gaming. PC gamers simply don't use that resolution.