It's extremely easy as long as your motherboard is willing (as in, it's based on a decent chipset and the BIOS has all the options you need). With the 6400 I think you can expect a 50% overclock rather easily (on air) and perhaps something like 70-80% if you're fairly lucky and good cooling.
By the way, I think your scale messed up. If you ask how HARD something is then, obviously, the higher numbers should indicate a higher level of difficulty. If you wanted to ask how easy it is then your scale would be just fine. I'm just nitpicking because there isn't much else to say about overclocking and I felt my post was too short
EDIT: Forgot to close my parentheses, oops.