http://www.netstumbler.com has most everything you'd need.
orinoco cards are the best, but others work see the stumbler forums for which, other than that just run netstumbler and drive around, you don't need an external antenna but it helps. it'll list the available wireless network points etc.
depends on your laptop, most have cig lighter adapters, simplest (but sometimes a bit more expensive) is to get a power inverter and plug your laptops brick into that.
a gps is needed if you want to map out the area, radioshack(tandy) are having a closeout on the digitraverel gps for like $45 it works pretty good with a pocket pc, i am adapting mine for stumbling., otherwise you just wander around and note locations by hand.