We do need some kind of comprehensive info here for comparing options eh.
HDD size... remember that if you get one that does DVDs, you will probably be recording to DVD pretty often. I know I've had my Humax DVD for a year and have made hundreds of DVDs with it. I only keep content on the HDD if I want to watch it and delete, or if i havent gotten to putting it on a DVD yet. Mines 80gig, but you will be OK with a 40GB just the same until you decide to upgrade the HDD.

Also keep in mind that at Best quality, you only get 1 hour on a DVD of video. (High = 2, Medium = 4)
HD content... you should be able to run a S-Video line out of your HDTV tuner box (cable box) assuming it allows multiple outputs to run the content at the same time. Then just run the S-Video into the Tivo unit for recording. When watching HD content, just watch it directly off the cable box via the component/hdmi/etc. Wont record in HD but will let you record the programming.
The Series 3 units have HD support but aren't out yet. However there are options for HD DVR content off cable out now.