The 940 and 955 are both "Black Editions", which means that they have unlocked multipliers, like the old Athlon FX series...
The 940 is 3.0ghz and 955 is 3.2ghz.
Most CPU's allow you to lower the multiplier, but not go higher than it's max locked multi...
For example, having a CPU with a multi of 14X gives you a clock speed of 2800mhz (14X x 200, multiplier times FSB). To overclock, you must raise the FSB, so then you'd have like, for example, 14X x 220, which would give you 3080mhz... BUT, the catch to this is that most motherboards will also raise the memory speed, NB speed, PCI-E/AGP speed, etc... which causes instability.
The new black edition CPU's allow you to leave the FSB (and also the rest of the system) at 200mhz, and raise the multiplier... so if you have 14X x 200 = 2800, you could try 16X x 200 = 3200mhz, and the rest of the system is left running at spec speeds.
I know there's much more to this than that, but maybe this will give you a better idea of what to buy if you're looking to overclock.
