dazdaz, yes you can use that card, the latest map for a Go300 is only 118mb, so a bit of a waste. The latest map will cost £39.95 and is available to download, you'll probably have to do it from TomTom home though.
matt_hooks the latest TomToms have an overspeed warning that can be set to remind you that you have gone a bit over the speed limit. Most people don't deliberately go out with the intention to break the speed limit, it's so easy to wander over. I think the speed camera warnings actually prevent a lot of speeding and are a good thing.
Tosh, yes they are acurate as long as you are not on a major incline, car speedos can overread but never underread (from build and by law)