Your cable box is outdated so I would suggest you contact Comcast on Monday and ask for a new HD box with HDMI output. You shouldn't be charged anything not even monthly. Your HDTV has 3 HDMI input (2 in the back and 1 on the side), HDMI will give you the best quality.
If you record a lot, it might be worth to upgrade to HD DVR for an additional $3-4 a month, or ask if Comcast have special promotion in you area (free upgrade). I don't know if Comcast bundle an HDMI cable but if not you can get them for cheap here, $6.99 a cable or $8.99 for two with free shipping: http://www2.meritline.com/6-feet-hdmi-1-3b-to-hdmi-1-3b-a-v-cable---p-26853.aspx. This special deal doesn't run everyday so get it while it's available.
I can't find the specs (input-output connectors) for your Toshiba DVD/VCR so I can't comment on the best settings.