If your Tivo only contains one hard drive, you can upgrade it by merely adding another. Of course this voids every warranty the thing's got, but Tivo doesn't seem to officially discourage it.
The process boils down to:
Selecting a good hard drive (must be fast enough to avoid compression artifacts)
Format and Bless the drive, so Tivo can recognize it (done with downloadable utility)
Backup the original Tivo hard drive (optional)
Installing the drives (which requires some bracket fabrication, or buy one from 9thtee.com)

There's about a million that boast Tivo hacking tips, here's one.
http://www.cnet.com/electronics/0-3219398-8-4562148-1.html

BTW: A friend of mine saw my Tivo and described it as "Tivo... It's all your shit, only different. "