I am asuming that these are freeware music videos, not copy righted stuff as that could not be discussed here (since they are downloaded). Unfortunatlly, If you convert to dvd compliant video, you are not going to get a lot more room (some but not a lot). Dvd uses mpeg2 so you are going to have to convert from mpeg to mpeg 2, which is going to take more space but not improve quality over the original.
To do this, open start smart and find the "make your own dvd-video" icon. Just clcik add video files and keep adding video files till it is too large for the disk, at which point it should ask you if you want to reduce quality to make it fit. Select yes and see how much it will let you put on thier. You do have to be below the 4.38 gig on the bar at the bottom before you continue though (go past it to get it to kick the quality down). On the next pages nero will let you create simple titles and buttons for each video file. You can use a video sample from the files for the button images and or background image if you like. You can of course clcik the more button, click the video options, clcik the dvd video tab, then change the quality settings. I would think that since you are coming from lower quality video (mpeg1 I am guessing), you should be able to go to super long play and still have quality as good as the original, but you will just have to try it and see which qhality setting gives sutable results.
Another option, if you don't mind wasting a disk to try it, use the nero smart start, create data dvd and just add mpeg1 files till it is almost full. Some dvd players can read mpeg data dvd's and that would allow the most content (a few will even play xvid etc. but not too many). You won't get menus but you will get the most content if your player can read them.