What SD Card you need depends on how much stuff you'll want to install or store on the card. The come in a large variety of capacities and anything from 256 megs to 2 gigs is commonly available. You likely won't need 2 gigs unless you'll be carrying large databases such as medical references or a few full length movie. We have reviews of several brands of cards on our site. Getting a super high speed card isn't too important unless you plan on watching videos from the card.
Dell sells spare batteries should you need one and Mugen is another popular brand which tends to be a bit cheaper and higher capacity.