Depending on the light, you may find that rechargeable cells with an equivalent capacity actually perform better in your light, especially fairly modern LED flashlights, many of which are starting to be designed around rechargeable cells for their better current delivery characteristics.

Sanyo Eneloop rechargeables (or Duracell 2000mAh "Pre-charged" rechargeable cells -- these are made by Sanyo with a Duracell wrapper on them) as compared to a run of the mill 2000mAh drug store alkaline cells will in my lights put out a bright, constant, light for more than 2 hours while the alkaline starts to dim dramatically after 30 minutes.

In fact it is safe to say that any good quality NiMH rechargeable cell will outperform standard disposable alkaline cells of equal capacity, and in many cases will outperform those with much greater labelled capacity.

Same capacity. Different delivery.