Well this is my interpretation of a geek and a nerd.
A nerd is someone who is very proefficent in a certain field of expertiese. (i.e. computers, mathematics, and yes even sports can make you a nerd)
A nerd is not generally a bad thing I think, it just generalizes you a someone thats spends much of your time to something and your good at it.
A geek, to me however, defines someone as spending too much time on something but not generally good at doing whatever they spend their time on. For example, a computer geek loves to spend his time playing pc games and such but dosent know how to operate anything else other then the disc port, keyboard, and mouse.
It is possible that nerds can be geeks but I dont think geeks generally become nerds. (still possible just not often)