I did 2 things:

1) Installed Sveasoft's firmware on my WRT54 and boosted the output power to 150mW instead of the default 28mW

2) Installed the dual external Linksys antenna's which are spec'd for a 7dBi peak gain.

These antenna kits are simple off-the-shelf items at Microcenter and Fry's.

For your router I am not so sure there is a version of Sveasoft (or OpenWRT for that matter, another alternative), but here is the "official" Linksys antenna:

http://www.linksys.com/servlet/Satel...VisitorWrapper

which was in the list posted by the previous poster. FWIW; Fry's DOES have the largest selection of additional antennas I have ever seen, including many really large ones, external (outdoor) ones and the famous "cantenna". Compusa only carries the linksys ones, and one or two Hawkings.