In my view the easiest to install, and cheapest is probably an Orinoco card (or clone) off Ebay. They are fully compatible with netstumbler, and with patched linux drivers you can kismet as well.
For antennas, generally speaking you get what you pay for. You can get cheaper 5.5dbi magmount omni antennas, which are good if you are on a budget and do improve the signal, but the best one omni is the magnetic mount from fab-corp (
www.fab-corp.com) that has a built in ground plane, but costs significantly more.
As for directional antennas, I don't have a huge amount of experience with them so perhaps someone else can advise...
BM