HD Cable Setup with a Series 2

I have a regular Series 2 TiVo and am considering getting a HD Box from my company Adelphia. Does anyone know if a HD Box will work with a regular TiVo so I can still record non-HD shows?

I have a Sony KP-53HS10 which is a standard 4x3 screen and HD Capable. I'm really happy with my setup where I have the cable split going directly into my VCR and my cable box. The cable box sends the signal to my Tivo and my television, and then my Tivo sends it's signal to the VCR and the TV. Saying all this, is it worth switching to a HD box? Will it effect my setup (much)? And then, will the picture be worth it on my Sony TV?

Thanks for your help!

Larry