Upres is not going to make the look better, possibly worse. The video itself is still the same low resolution. There are some powerful but not so cheap software that can clean the video. iMovie can easily crop. I don't recall off hand if you can set a new movie to a specific resolution and screen size and drop clips into that timeline. That upconverts and crops (one of the preference settings). FCP can do that. You may be able to first drop a high resolution clip with the screen size you want, then the low resolution clips then delete the high resolution. I don't use iMovie much.
