The Hinsdale How to Upgrade Guide works fine on SA (Standalone) UK Tivo's which come with either 30G+15G or single 40G drives. You will need a Torx T-10 driver to get into the Tivo. I went for replacing with a single 120G 5400 rpm drive. Chose a quite low power drive, performance doesn't matter that much and slow low power drives are probably more reliable. You can fit 2 drives (but twice as unreliable and you may need an extra bracket). You can fit larger than 120G but you need to hack the kernel which I haven't tried. Note: the upgrade guide refers to command like 'msfrestore' but the latest tools seemed to have been rolled into one executable with a parameter e.g. 'mfstool restore' followed by arguments.
The cache card is from 9th Tee. The consensus seems to be that fitting the cache memory probably isn't worth it if you have a standard drive but becomes more worth while on larger drives. I have fitted 512M on my 120G Tivo and it helps a lot removing most of the sluggishness from the UI which is my pet hate. The drivers are installed via a PC onto the Tivo drive. You use Linux to do the install booting from a Linux CD the ISO for which is here. With the drivers installed you can now telnet to the Tivo and get a BASH prompt. You can also FTP to transfer files.
There are some standard Linux tools compiled for the Tivo here. The standard Tivo bin directory doesn't include even basic commands like ls.
I installed the Tivo Web server software here remember to replace the UI module with the UK version (which is in the distribution) otherwise things don't work properly. There is a more advanced version here which I haven't tried yet.
There is an FTP server here that allows access to the media streams (rather than the Linux files you get with the standard server). The two servers run together on different ports.
I use Apache as a reverse https: proxy to present Tivo Web to the internet (documented in the Tivo Web read me). Another option is here.
Other links: