Upgrading the dir-855 router?

Firstly, a firmware upgrade is most likely not going to solve your range problem. Secondly, unless the router has detachable antennae (which most new low end routers don't), you won't be connecting a high gain antenna to it. As far as distance, if you really want to reach out and touch someone, you could spend a small fortune on a bi-linear wifi amplifier (get out the charge card because it aint cheap). That also still requires a router with detachable antennae.