If you use xchat, you wan't to try this!
$ sudo aptitude install festival festvox-kallpc16k
get this simple script
$ cd ~/.xchat2 && wget http://vorian.org/sayit2.py
go under your xchat tab > load plugin or script > places: .xchat2 > sayit2.py and load it!
enjoy (the script will cause your computer to speak only when someone triggers a highlight)