Morning Pammy,
The unit comes with a 128mb card which is big enough to hold all of England *OR* the Major Roads of Europe, which is provided on the CDROM in the box.
If you want to load them in parallel, you'll have to buy a 256 or bigger. I picked up a 512 in Singapore for £30.
The detailed country maps for Europe are a separate package that needs to be purchased, or knicked from a popular download service on the internet.
The card is used for everything - the whole operating system appears to boot from the card and the unit will do nothing without a card installed. Maps and POIs are stored on the card too.
The camera database probably needs some clarification:
TomTom have released a software update (you'll need to download and install from their site) to enable custom point of interest to be installed, so the kind people at pocketgps.co.uk are merely providing a data file containng the co-ordinates of the speed cameras, specs cameras and red light cameras. Becuase of this, the files are ever so tiny (all databases total 530k - impressive).
Site is here:
http://www.pocketgps.co.uk/uksafetycameras.php
As you can see, there is no real need to use a bigger card. The software that comes with the unit allows you to add/remove maps and voices from the card via the USB link cable provided.
Let me know if you would like to know any more!