I do receive some HD channels and they don't look any different from the regular channels. Question: If TiVo is recording an HD program, is it using more disk space than a regular program, or is the space it takes dictated strictly by the quality level set for the recording?
Series 2 Tivos aren't HD Tivos, so it's not storing it in HD and you wouldn't be seeing it in HD either. Series 3 Tivos are HD (that includes the Series 3, the TivoHD, and the TivoHD XL).
These days, frankly, you're best-off getting a TivoHD and asking your cable company to send a guy out with cablecards. You might have to push a little but they should be required to give them to you. I think that would solve a lot of your problems.