Electrical schoolwork...

[Chris Thorp]

Just to clear up any confusion, Ti's answer is the technically correct 
one.  Unfortunately, the "resistance" of an LED is essentially a 
meaningless number because of the nonlinear nature of the device.  IE: 
there is no "ohms law" that applies to a diode (or LED) directly.  For 
LED performance, the current that flows through the device is all that 
matters.  The "typical" voltage across the LED is supplied to assist 
with the selection of the proper dropping resistor.

The two specs needed to properly drive an LED with a series resistor 
are: the nominal (or typical) forward voltage drop and the typical 
operating current.

Summary of Ti's calculations:
1.
[(supply voltage) - (nominal LED voltage)] / (desired current) = (needed 
resistance)

2.
[(supply voltage) - (nominal LED voltage)]  * (desired current) * 
(safety margin) =  (needed resistor power spec)

(Note, all voltages in volts, currents in amps, resistances in ohms, and 
power in watts)

Good luck with the LED upgrade! 

-Chris