Can't really help with the maps problem, but as for the memory card, you need a microSD card so are pretty much limited to 6GB right now.
They are generally about the £35 mark
http://www.google.co.uk/products?sou...&q=6gb+microSD
EDIT: having said that, here is an 8GB one:
http://uk.insight.com/apps/productpr...05HBQ&src=FRO1
Although interestingly, it doesn't appear on the SanDisk website for some reason.