Hello Jay,
There are many combinations available, and that makes answering your general question difficult.
2 CR123 cells will give you 6 volts. A 17670 cell will give you 3.7 volts midway through the discharge, but the range is 4.2 volts down to 3 volts.
If you have a direct drive light, the higher the voltage, the brighter the light will be, within the operational parameters of the lamp.
If you have a step up regulation circuit, the light may be just as bright on 3.7 volts as it is on 6 volts.
An example would be the SureFire U2. The early ones were able to hold an 18650 without problems. On 2 CR123 cells, the light is brighter, but just by a little bit, however it will run longer with the 18650.
Runtime is directly related to capacity. In the example above, the CR123 cells have roughly 1500 mAh capacity. The 18650 comes in at 2600 mAh which is roughly 70% more.
If you have a specific question about a particular light, chances are someone has already asked about that light. You can use the search function to find what works in various lights. You may have to search the General Flashlight, LED, and Incandescent forums as well as this forum. You may also find interesting combinations in the Homemade and Modified forum.
Tom
