If you've got a fairly recent navcore installed it will have included a bootloader that enabled the use of SDHC cards so ,in theory, up to 32 gb will work although I've limited myself to 16gb and that works fine..
You have 2 choices when using a card...just extra maps for the software (navcore) within your TT to use OR , a completely different navcore along with whatever maps you want to use besides those on the TT's memory and in that case your TT would automatically choose the navcore on the SD to use until you removed it at which point it would use the TT's own navcore...(that's not to say you can't have the same navcore duplicated but what would the point be?)
There are also 2 types of camera files...tomtom's own type (of which there also 2 types...premium or standard) and aftermarket ones supplied by companies other than tomtom in the form of POI's. Either sort do become less accurate after a period of time and benefit from being updated although I doubt whether either sort was included in the map you downloaded..
have a read through the FAQ section for more on cameras where you'll see that the TT cams need cracking (just like you did with that map) whereas the POI type just get added to the map and then you set alerts for them from within the TT's menus.
I'm running navcore 8.351 (9982/xxxxxx). Would there be any benefit to getting a more recent one (if there is a more recent one??).
And just to pick up on your advice "You have 2 choices when using a card...just extra maps for the software (navcore) within your TT", this sounds like what I would like to do, but does the tt not ignore what is on the tt itself when a sd card is inserted? In which case there would be no files (navcore etc) to run the satnav and load the maps? Or am I off the mark, and the if there is no navcore etc on the card it will look for one on the unit itself and use that one, while still giving access to maps of the sd card?