Speaker db rating?

Does anyone knowthe ratio of a speakers db rating, 1 db at 1 meter,relitive to amplifier watts to power them well enough to be loud enough in a small room.
Example:what kind of db rating would one look for if a 8 watt amplifier was used?
How does each point,as the rating increases,affect the wattage needed to drive the speaker.
Thanks
G.H.