I'd recommend against the 2GB card unless a) you enjoy paging through 100+ screens to find the title you're looking for, b) plan on carrying several very large PDF books, or c) plan on carrying a lot of music/pictures with you. I started with a 512MB card and put maybe 35 titles on it, plus several others on the internal memory; this made it a little difficult to find what I wanted, esp. if they aren't native LRF format and have meta-tags. I am now to the point where I don't really use the SD card for books unless I don't want to have to sync the Reader w/ my PC.
Other than that, while you're waiting you might want to fiddle around with some of the conversion software you can find in the forums, as I'm sure you'll be looking for them in the future and you can start playing now.
: Here's a link
to a thread about collections and external memory cards. While it doesn't specify how big the card is, it will help clarify what I was refering to about too many books on a card. I kinda like yvan's idea with multiple cards, but I'm not that OCD.