Get yourself some decent Parabolic antennas and make sure that you take into account the curvature of the earth and elevation of both locations when you make your shot. You will also need to make sure there is no foilage in the way due to the fact that 2.4GHz is the same frequency that water resonates at (think microwave ovens). If there are trees in your way I'd go down to 900MHz which would be a little slower but you'll be able to do longer shots with it. I've been able to do 25-30 Mile shots on two senao pcmcia 200Mw cards a 24dBi antenna at one end and a 16dBi yagi on the other. One more thing you may want to consider is to use high grade cable such as LMR-400 or better. If you need any help, PM me an I'll offer you some assistance.
Here's some links that you may want to check out:
HyperLink Antennas
Seattle Wireless
Personal Telco