Your Nokia 5800 has the option of merging the videos that you have on your phone. Here are some steps on how to merge the videos,
1. Go Gallery -> Images & Videos
2. Select any video and click options.
3. From the options select Edit where you get the options like – Merge.
4. There are some other options too like change sound, add text etc which you can give a try.
5. When you click Merge you get another screen where you have to select another video or image into the video you have already selected.
6. Now click Video clip and click another video, when this is done you will be asked to merge whether this has to be merged from the beginning or end.
7. When this is done the two videos are merged and you will be asked to enter a name for the new video.