What SD memory card do I buy?

I need to buy an SD memory card

What do I buy?

1GB? 2GB? 4GB?

What make should I buy?

Kingston SD? Sandisk SD? Toshiba SD? Panasonic SD? Transcend? PNY?

But then I have to decide what speed I should buy!?

AND: should I buy a cheap one from a foreign seller on eBay?

I've bought a Satnav and want to upgrade the memory + install newer and better software + install new maps