Just for a rough idea, in our installation, I'm covering triple that
distance with two panel antennas. One is an effective 10 (after cable
loss) dbi and the other is 14 dbi.
The AP is a Buffalo HP and the client is a Linksys WRT54G. I'm
getting about 40 db S/N reading in the firmware on the weaker
(Linksys) end.
In other words, I've got more antenna than I need. You could get by
with two 12 db antennas with just about any radio, I'd venture to
guess.
Might even work with a stock omni on the AP and a 14-19 dbi panel on
the client. Or vice versa.
If you really have a clear fresnel zone and all that.
But still, do the link calcs.
Steve