I believe you need to look at all the components to get one whole overclock.
1. The motherboard - lower multipliers require high FSBs. Intel P965 chipset boards do very well here, but beware the strap-changes that often seem to happen that makes CPUs running between 400 and 455FSB run slower than ones running under 400FSB!
2. The RAM - Many boards require the RAM to run at a fixed ratio. This quite often means that PC6400 RAM is the minimum and to optimally overclock a low multiplier CPU, you need really good RAM. Expensive RAM.
3. The cooling - high FSBs place big demands on all the chipset components so the cooling of the NB and SB can be critical to getting a stable overclock. All the newer Intel chips all seem to suffer significantly higher core temperatures than the very early ones, although the reason for this is unclear, it is definitely the case that many have concave Integrated Heat Shields (IHS) and some have poor contact between the cores and the IHS. E4300 CPUs also seem to need lots of volts to get them overclocking above 3GHz so CPU cooling is a factor too.
4. The CPU
For this, I think you need to set a target value you want to overclock to. I like to pick 3.2GHz, because it suits my argument. I make no bones about that, and I welcome the opportunity to discuss my logic.
E6300 with 7x Multiplier requires FSB of 457 to reach 3.2GHz. Sp PC6400 RAM will also be overclocked and PC8000 RAM is required to guarantee the RAM will match the CPU capabilities. A voltage increment will almost certainly be required to get to 3.2GHz.
E6400 with 8x Multiplier requires only 400FSB to reach 3.2GHz. PC6400 RAM is not required to be overclocked, so the potential to go further is also there. A voltage increment might be required. Some E6400's don't need a voltage increase to get to 400FSB.
E6600 with 9x Multiplier requires a mere 356 FSB to get to 3.2GHz so the PC6400 is actually underclocked at that speed. Generally, E6600's will do 356FSB on stock volts, or just a little over stock volts.
At this point the E6300 will be the system producing the most heat and require the greatest cooling. The E6400 will also need good cooling, but less so. The E6600 is probably running quite cool.
Now, if you throw in the E4300 then it should enjoy the same low FSB advantage of the E6600, but it's starting from a lot further back and it needs lots of volts to get there. It produces lots of heat and therefore needs loads of cooling.
I think the sweet-spot is the E6400 as it offers a near-guaranteed 3.2GHz with ordinary PC6400 RAM, has a little more available headroom than the E6300 to go beyond 3.2GHz and it only costs £25 more.
