hi there
if you just need a quick&dirty serial connection to your wrt54gs/wrt54g without any capacitor or resitor just use cmos logic. the modern tty/com/rs232 interfaces support 0V/5V voltage levels so there's no need for a max232 or anything like that.
the clue is: linksys uses 3.3v on their boards: you can use this for a 4001 or 4011 cmos as input and the cmos gives you 5V output which is not 12V as required but enough for most rs232 interfaces. a 4001 or 4011 is a standard logic and is very very cheap (below 1€)
the thing to do is: just invert the signals from the linksys and "upgrade" it to 5V. you can use nand/nor/intverter logic, just what you have in your drawer ...
http://bugs.langhofer.at/viewtopic.php?t=84
for questions just ask here!