hey,
The answer to this depends on which version of the router you have versions 5 and 6 can be modified but are not as good as the earlier models due to less memory and other resourses in the new models as well as the switch from a linux based unit onto the vxworks O/S.
Do a google search for linksysinfo and have a look through the site it will guide you in what options are available to you.
I myself have 3 wrt54g's and have stuck 3rd party O/S on each and even have a webserver and ftp server setup on one router by adding a sd card to give more space. the other 2 have a wds link between them to extend my network even further.
Bob