Well, with overclocking, you want to go slowly. Get ahold of [google]Prime95[/google] first. Also, have [google]Memtest 86+[/google] handy as well. Especially if you start tweaking the memory timings and such, Memtest is excellent for hammering your RAM to see if there's issues.
For the CPU, use one instance per core, although if you have HyperThreading, you'll want to have a second instance for that. If you can't run it for 24 hours without any error, you're not quite stable. Up the voltage by the smallest increment available, test again.
You'll want to run Memtest for a few hours, but if you can, do it overnight to give it a good beating.
Voltage will give it an extra kick to keep the singaling going, and this applies to any overvolting of an IC. Where you run into problems is when you put too much voltage in that it can't be cooled enough, or you have too much voltage going through that it damages the chip. I'm not sure what the max safe voltage is for the Pentium D's, but I do know that the Northwood core (a very old P4 core) had a nasty tendancy to die when given 1.7V or more, regardless of what cooling was used (air, liquid nitrogen, etc).