I just replaced the string nuts of my guitar and bass from crappy plastic ones, to a "tusq" nut on the guitar and a graph-tec nut on the bass. It made a lot of difference in the tone of both instruments, And I recommend to anyone who has a plastic string-nuts on their axes to change them immediately to either bone, brass, nickle/steel (to make open strings tone blend in with frets better), or the above mentioned graph-tec ones. To remove old nuts, tap firmly, but lightly, on the old nut in a downward direction with a hammer to losen it from the fret board. Then file the new nut if neccessary (many are pre-filed or in need of only resizing to fit into its slot). The nut should been held in by glue, but it isnt always needed if it fits the slot snuggly.
Most factory guitars in the low-to-medium end come standard with plastic nuts, so if you think your guitars tone is being sucked up by a nastly, cheap plastic string-nut, replace them and enhance your tone. I know to most of you this is somewhat elementary, but its often over-looked.
-Fluke