As far as performance, they should perform similar when clocked at the same speed. The newer ones may run lil cooler (binned better), but thats about it.
The prices on the other hand, is due to supply and demand. Since they are phasing out the older E6xxx 1066FSB CPU's, what is left would explain the higher prices, and perhaps Intel wants to get users on the rated 1333FSB.
All the E6x50 are 1333FSB, all the E6x00 are 1066FSB.
All four CPUs you mentioned have 4MB, so go with the higher FSB.
The E6750 is less expensive than the other three, and it's usually easy to get it to run at 3.0GHz.
I don't like to OC.
OC makes older the CPu's life with 700 % . So no OC for me.
This may be true, but the Celeron 400 that I overclocked to 500 was still chugging along about 5-6 years after I built and overclocked it. I only stopped using it because the system was outdated. My mobile Athlon XP 2600+ overclocked from 2.0 Ghz to 2.5 Ghz for years. It outlasted the mobo and a couple PSUs along the way too. My E6400 has been overclocked from 2.13 to 3.2 Ghz since September 2006 and is still running strong. I don't doubt that overclocking shortens the life of processors. More than likely, though, the processor will be outdated before the life would run out of it anyway. Both my Athlon and the Core2 were overclocked with increased voltages as well.
In addition to those mentioned I've overclocked another Celeron 400, an Athlon XP 1700+, a Mobile Athlon XP 2400+, a Barton Athlon XP 2500+, a Sempron 3000+ for shorter periods of time and have never lost a CPU as a result.