Here’s a guy who manages to modify his Linksys 54G router into a super router by installing a CPU heatsink with an external fan, adds super-long 9dB antennas, and aftermarket firmware DD-WRT to overlock the darn thing as much as possible.
I actually personally using one of these routers with DD-WRT, overclocked a bit, but this hack certainly will allow you to be on the very bleeding edge of router hacking.
Also, Linksys 54G series might be old but they are the cheapest and best routers that I wished Linksys would keep making. Of course, you can probably do something similar with the newer Linksys routers. (see my best Wireless N Router Review to see which ones support DD-WRT)