What free DVD burner software can I download that fits my computer'sspecifications?

You may need to get help on iDVD on the iDVD forum. You are using an old version and may find very limited support with it since I suspect most people will look at what you are trying to do and think, "Get a newer computer nad a newer version of iDVD." Not much help I know but that ill be their reaction. It taking 7 hours to complete (most of that wasn't the burning but the processing of the video into DVD format) is a sign that asking iDVD to encode a full movie on an 800MHz G4 with only 700MB RAM is asking a lot. It is possible something failed midway. Yes, it should have just taken longer, but things don't always work out logically.

If this is a commercial movie you probably should just consider going the (legal) route and buying the DVD. In terms of wasting a bunch of DVDs and hours trying it may be cheaper+easier.