Hi mate - a litlle advice:- I found my 4200+ to be very memory bound - high grade, fast memory will generally allow the 4200+ to clock up better, as the increas in CPU speed also means an increase in memory spped.
http://forum.overclock3d.net/showthread.php?p=171049 That URL takes you to my thread on OC3D - I can clock my 4200+ up to a hefty 3GHz on air cooling.
Remember that the OC a chip is capable of is unique to each rpocessor - some may not clock as well as others.
My advice would be to up the voltage to around 1.4 volts, up the northbridge and southbridge voltages, whack your cooling up to high and start clocking bit by bit.
Keep the multiplier at 11x and move up the HTT link speed slowly in stages to find exactly how far you can get.
Anyways, read my topic on that link and keep me posted.