Just about any microphone on a boom (a long pole) is better than using the mic. built into your camcorder. Directional (or shotgun) mics. are usually the best choice. (See the Knowledge Base for more info on microphones.)
Even the cheapest audio hardware that comes standard with most computer systems is adequate for the amateur film maker. There are higher quality audio cards that can be added to your computer, but you should only consider them if you’re really picky about the quality of your sound, or are looking to pursue a career in the audio field.
On the editing side of things, I find that most decent video editors (Final Cut, Avid, Premiere, Vegas Video) have enough audio editing and mixing tools to keep the amateur movie maker happy.
My audio knowledge is a bit weak, so if anyone wants to provide more newbie info please send me an e-mail at firstname.lastname@example.org.