With the gear you have. AMplifiers are not the way to go. You need a good antenna. Trees are rough becasue the leaves contain lots of water and water absorbs 2.4gHz signals very well.

Are you looking for a new internal card for your laprop, or pcmcia or even a usb? You will need one with an external antenna connector (a jack).
Are you looking for dirt cheap or willing to spend a bit of dough? I can google a few suppliers if you wish.

For best signal, you will have to have a direct view of the accesspoint you are wishing to connect to. This is called 'line-of-sight' or LOS.