Keeping within EU EIRP limits

Hi all,

The legal limit for EIRP in my country is 100mW or 20dBm.

I have a 2km wifi link between 2 roof tops in a built up area, nearly
line of sight. (speed is about 0.3 Mbps)
I am using Linksys WRT54's with Alchemy firmware and D-Link ANT24-1800
18dBi square panel antennas.

>From what I understand, using a higher gain antenna would be helpful
even though I have to decrease the transmit power via the router web-
config page to stay within the EIRP limit, because the higher gain
antenna provides higher receive signal levels.

Is this correct?

Right now, my setup uses 2 of those 18dBi panels per WRT54, (this
router has 2 removable antennas).
If I were to buy higher dBi antennas (24 or 30 dBi) to improve link
speed and relibility, would it be advisable to install the large
antenna only for the receive side?
(the firmware I use allows one to specify the Tx/Rx roles of left/
right antennas or "auto".)

So in order to save money I may want to get just two new large
antennas, instead of replacing all four.

Any thoughts?

Thanks

- arraydent