overclocking is such a tempermental thing...it varies depending on the mobo you use, the chip you have, the stepping on said chip, the cooling you have etc. i just built a system with the same cpu and i use an msi board and the msi corecell program lets me change the fsb and voltage on the fly or i can go into the bios and turn on the Dynamic Overclocking and let the mobo decide when to overclock (i.e. under a load) or not.
AMD X2 4200+
MSI K8N-Neo2
evga 6800gt
2gb (4x512mb crucial ballistix)