You should be able to get 1.5-2 hours on a 4.7GB DVD and still get good quality; I do 1.5hrs on mine - 1 episode of MST3K each.
For shows I've recorded with my Hauppauge PVR-350, I use Ulead Videostudio to edit and encode them to DVD-compliant MPEG-2 with Dolby Digital audio.
For MST3K episodes I download (hey, most of them are not even available for sale at all), I use Virutaldub to flush them out to Huffyuv (lossless format), as some of the files have errors that Videostudio can't read past, but Virtualdub can. I clean up the audio, and then use Virtualdub to put the audio and video back together, maybe use a few filters to improve the video, and then use Videostudio to make a DVD compliant file again, and then burn it to a disc.
I get pretty good results that way - very-near-to-TV-quality on the stuff I record myself, and maybe VHS quality (SLP) for the downloaded stuff. Of course, the downloaded stuff has been resized to 480x360, with square pixels, rendered with 1990's quality codecs and PCs, heavily compressed, then finally resized again back to 720x480 non-square pixels and stashed on a DVD.
And with Videostudio, you can adjust the output bitrate until you get a file that's within 10MB of 4.7GB - I try to get very close, leaving maybe 2MB as space for a menu.