Rocket:
I don't know exactly how much storage memory each country requires, however, if it helps you, I normally use a 512 meg CF card, and that lets me store just about everything I need for any one long tour. It is also enough storage to allow me to load it (selectively) with South Africa and Canada, in addition to my home base area in Europe, when I go to those countries.
Don't forget that you will get a CF card included with your GPSR. I think the current shipping product is a 256 meg card with the European SP 26xx units. You can use that card for extra capacity. By example, you could load the British Isles on the 256 meg card, and then load whatever regions of the continent you plan to travel to on the 512 meg chip.
If you want to be sure you can load more maps that you will ever need to use on one tour, you could buy a 1 gig card. But personally, I think the combination of the included 256 meg card, and an aftermarket 512 meg card will be more than enough for you.
When you buy an extra CF card for your GPSR, buy the absolute slowest and cheapest name brand* CF card you can find. Speed of access of the card means nothing to your GPSR, speed is only important if you are using the CF card in a digital camera. The GPSR reads the CF card at boot, and then only rarely reads the card as you ride along, so don't waste your money on 'expensive' super-fast cards.
PanEuropean
* By 'name brand', I mean get a card made by a recognized, trusted manufacturer who has a reputation to protect, such as SanDisk, SimpleTech, Lexar, Kodak, or similar.
