Each chip is different. One thing for sure, though, is that they run ridiculously hot, so you'll need something significantly better than the stock cooling to get moderate overclocks (and possibly ANY) to keep temperatures within the spec. Most of the Pentium D's were already at their thermal limit at stock levels, so keep that in mind as you go.
You're going to have to experiment with settings to find a good overclock. Lower your multiplier and then begin raising the FSB and testing with Prime95 or OCCT to find out how much your chipset can handle, then start raising your multiplier by one notch to see what the limit on stock voltage is. Then start upping the voltage and multiplier to find the limit, with each step stress testing to check for stability. Be absolutely sure your temps are ok too, you can use a program called SpeedFan to check those. I wouldn't go over 80C, and I wouldn't use more than 1.4v.