Unless you're capturing digital broadcast, half D1 should be adequate. In theory, each re-encode diminishes quality, so ideally you'd capture at a bitrate and running time that would nearly fill a DVD. If you're going to re-encode anyway, I don't think it matters much, so long as you never go from lower to higher. And I suppose your capture card only does full D1?
I'm not familiar with Roxio DVD Builder. Will it allow you to compile an oversized project? Oh, and will it accept elementary streams, e.g. m2v video and ac3 audio? If the answer to both questions is yes, here's what I'd do: Capture at 6,000, that's more than adequate. Convert the audio to AC3, and demux the video stream. Author 4-5 hours of your MPEG captures. Re-encode at half D1 with DVDRebuilder.
Now, if you don't want to re-encode overnight, just capture at proper bitrate, then convert the audio to AC3 and author 2-2 1/2 hours worth and be done with it. Blanks are cheap nowadays.
A bitrate calculator will be useful to you: