Link power-requirement estimation tools?

Does anyone have a pointer to a good one available on the net?  Given
distance and power, what's the right way to calculate the right (and
legal) amount of gain you want from an antenna?  For instance, using a
pair of 200mw cards, what would be the right antennae to use for a 0.6
mi link?  I figure that "too big" just means "harder to aim" and maybe
"more unsightly" -- but there's probably no advantage to over-specifying
the antenna, right?

Thanks,

/jordan