I've always envisioned nerds as being completely interested in math/science/etc. and NOT being sociable (this was the usage throughout my life as I was going up).  Nerds were those kids with no friends except other nerds.  Nerds play Magic the Gathering and Warhammer 40k.
Geeks are interested in math/science/etc., but they will at least try to refrain from making this obvious.  Geeks might play video games, or they may even partake in the occasional game of Magic in times of desperation.
I was a geek because I was always smartest among the people I would hang out with.  Every group of friends needed a geek for brilliant schemes and a loser to make fun of and take advantage of (but the loser and geek can't be the same person).  My school  was short on geeks, so everyone loved the few of us that were there 
