You could overclock it if you have suffficient cooling but, as your probably are aware, it's a pretty poor cpu.
Better to see what P4 M/B can handle and go for the upgrade.
The fsb is very important. if your m/b can handle a P4 HT 800fsb, go for it.
Intel Core2Duo 6400 @ 3.4Ghz
4GB Geil 6400 Ultra
3 x 500 Gb Samsung HD
EVGA 8800 Ultra OC