DVD video is based on MPEG2. So, yes, converting your Divx files to MPEG2 will give you about the same file size if you just added the DivX files to your project (because DVD Architect is going to go ahead and convert them to MPEG2). If you want to make a DVD that plays on standard DVD players, these are simply the paramaters you have to work with. DVDs use MPEG2 video, and MPEG2 video is generally going to use more space than Divx. On the other hand, Divx is not a standard for DVD video, so not all DVD players recognize it.

If you just want to put a bunch of DivX files on a disc and leave them as DivX files, you'll have to use a different program -- for example, Nero. (Though you won't get the menus and all the standard features of a DVD.) You will, however, only be able to use that disc on a computer or in a DVD player that recognizes Divx files.

The filesize of your Divx files is irrelevant. What matters is a) how long those videos are, and b) at what bitrate you want the final video rendered. You can lower that bitrate by going to File -> Optimize DVD and adjusting the default. Doing so will allow you more "time" on the DVD, but it will also lower the quality of your video.

By the way, the estimate in the bottom right of DVD Architect is probably wrong. It usually is. Given your situation -- going from Divx to DVD -- I'd recommend rendering all those files to MPEG2 using Vegas or some other method and then bringing the new files into DVD Architect. At least then you'd have a better idea of how much space you're really dealing with.

Rob