The "not enough memory" message is not referring to your 8GB of ram, that is plenty sufficient. The message means that you don't have sufficient free space on your destination drive to contain the movie that you are attempting to export. You would either need to free up some more space on your computer's drive, if that is the destination, or export the movie to an external drive that has more space.
An easy way to make a quality comparison would be to duplicate your project and, working with the duplicate, export a small representative segment of it at each of the two quality settings that you are considering. To duplicate your project, go to the project browser screen (where your projects are displayed as icons) and move your cursor over the project's title. A little circle with dots in it will appear to the right of the title. Click on the circle and a drop down menu will appear giving you the option to duplicate the project. Click on "Duplicate Project" in the drop down menu. A duplicate of your project will appear in the project browser screen. Open the duplicate. Choose a representative segment of the movie in the timeline and delete the remaining clips in the movie. That will give you a smaller sized project. Then export out the representative segment at the highest resolution setting that you are considering. You then will have a small movie that you can compare with your previous lower quality export. You can use the duplicate to test several export settings. You can create more duplicates by using the procedure described above. You want always to be working with the duplicates so that you keep your original movie intact. Making duplicates will not consume a significant amount of extra disk space because iMovie projects only contain small sized reference thumbnails, so you would not be duplicating the larger sized original media.
You didn't mention what version of iMovie and operating system that you are using. iMovie 9 sometimes has some issues with still images.
-- Rich
