I'd advice you to put up to 120 minutes per DVD-R and use the AVAMAT6 matrix. That way you would be able to use an average video bitrate of ~4700kbps and an audio bitrate of 384kbps (AC3 if you can, MP2 encoded via toolame as an alternative). It should look okay. Also, I'd recommend you to use CCE SP instead, in my opinion it's easier to use than TMPG and many people say its MPEG2 encoding is superior. Be sure to properly filter your video as well.