A Core2Duo CPU (Allendale/Conroe) are 65nm cores, so they will need lots less voltage than for instance 939 San Diego Core CPUs (90nm). Stock is 1.25, they are rated from 1.1-1.372 Vcore according to Newegg. Safe Vcore for overclocking on a good cooler should not exceed 1.55 Volts for 24/7 use, also because these chips tend to get extremely hot on higher voltages. Best would be if you stayed around 1.5 Volts maximum for day to day use, but in the end it all depends on the Temperature you get.
1.) Core Temp
2.) Intel Thermal Analysis Tool
E4300 = 1.80GHz - 2MB L2 Cache - 9x - 800 FSB
E4400 = 2.00GHz - 2MB L2 Cache - 10x - 800 FSB
E4500 = 2.20GHz - 2MB L2 Cache - 11x - 800 FSB
E6300 = 1.86GHz - 2MB L2 Cache - 7x - 1066 FSB
E6320 = 1.86GHz - 4MB L2 Cache - 7x - 1066 FSB
E6400 = 2.13GHz - 2MB L2 Cache - 8x - 1066 FSB
E6420 = 2.13GHz - 4MB L2 Cache - 8x - 1066 FSB
E6540 = 2.33GHz - 4MB L2 Cache - 7x - 1333 FSB
E6550 = 2.33GHz - 4MB L2 Cache - 7x - 1333 FSB
E6600 = 2.40GHz - 4MB L2 Cache - 9x - 1066 FSB
E6700 = 2.66GHz - 4MB L2 Cache - 10x - 1066 FSB
E6750 = 2.66GHz - 4MB L2 Cache - 8x - 1333 FSB
X6800 = 2.93GHz - 4MB L2 Cache - Unlocked - 1066 FSB
E6850 = 3.00GHz - 4MB L2 Cache - 9x - 1333 FSB
Q6600 = 2.40GHz - 2x4MB L2 Cache - 9x - 1066 FSB
Q6700 = 2.66GHz - 2x4MB L2 Cache - 10x - 1066 FSB
QX6700 = 2.66GHz - 2x4MB L2 Cache - Unlocked - 1066 FSB
QX6800 = 2.93GHz - 2x4MB L2 Cache - Unlocked - 1066 FSB
QX6850 = 3.00GHz - 2x4MB L2 Cache - Unlocked - 1333 FSB
Different Chipsets and their features:
As we are obviously most interested in the way these different chipsets overclock, let me give a brief outline what each of them is capable of.
The 965P Express Chipset is known for high FSB, but it has problems with D9 memory and the stock BIOS. Flashing to a newer BIOS usually fixes this, as well as the no-OC problem with some NVIDIA cards. For example the 7600GS, if you have that, it might not let you OC 1 MHz, but there is a fix. Getting a cheap PCI vid card and OCing with that, then putting the PCI-e back in will work for most people. As seen abvove in the table, it supports CrossFire, however only in x16 and x4 configuration (x16 PCI-e slot for the master card and x4 PCI-e slot for the slave card). Sources can be found in Post #5.
The 975X Express Chipset is not a fan if high FSBs, most won't even get to 400MHz (DDR2-800). This would be a good choice for CPUs with higher multi's (i.e. E6600, E6700 and Core 2 Extreme Processors). Most 975X boards are paired with the IH7R southbridge, which provides less SATA ports, an ATA channel but no Ethernet. If you want to do some serious overclocking, the 965P, 680i or RD600 chipsets will do better than this.
The nForce 650i Ultra/SLI is a great choice for someone who upgrades their computer to a Core 2 Duo, wants to do some decent overclocking but also wants to keep all his/her PATA drives. Yes, thats right, this chipset provides 2 ATA ports, which is not found on other Core 2 supporting mainboards. I can't tell how this board does overclocking, as the only board that is using this chipset that is out is the ASUS P5N-E SLI at the moment.
The nForce 680i SLI is THE overclocking chipset for NVIDIA grafics solutions. Numerous reviews and reports state that they have reached speeds on these boards that only the RD600 can keep up. A member of a German Overclocking forum got his RAM running at DDR2-1300 on a 680i board, stable at DDR2-1260. On the contrary, boards with this chipset only offer 1 ATA port but 2 x16 PCI-e ports and an extra 8x PCI-e port for a PhysX card or a third graphics card.
The ATI RD600 is the top overclocker board for ATI based grafics solutions. WindWithMe on Xtremesystems has tested a sample of this board here. Overclocking wise, it seems that this chipset is capable of world record FSBs and high CPU clocks, but that depends on the manufacturer of the mainboard (only DFIs version available at this point). If you are hunting after the highest benchmark scores, this board is not so much for you, as this thread over at XS states, see post 12 onwards. It supports 8x SATA, 1x ATA and ATI PhysX 2+1 Configuration (2x ATI CrossFire compatible cards in the 2x8 PCI-e slots and one PhysX Card in the 8x PCI-e slot).
Interesting Article about the different Southbridges: TomsHardware
DDR2-533, DDR2-667, DDR2-800 and DDR2-1066 is sold normally. For us Overclockers, DDR2-533 is out of question for serious overclocking, as it is only rated at the stock FSB of the Core 2 Chips. DDR2-667 is better, but for serious overclocking DDR2-800 or above is the kind of sticks you want. As of DDR2 chips, Micron D9 chips have premium overclocking capabilities. Top choices here are D9 Fatboy (not in production anymore though), D9GMH, D9GKX and D9HCD. For the D9HCD, they are specified at CL6 (Micron original at least) but will do CL4 at over 400MHz, results here. Elpida chips have mixed results, I can't complain about these GeIL DDR2-800 I have, they will hit DDR2-1000 with 5-5-5-13 timings and run up to DDR2-900 with 4-4-4-12 (stock timings). There is also chips by ProMos, Samsung, Infineon (Aeneon) and Hynix.
For more info, as to what chips your RAM has, check out this DDR2 RAM List. Also, for those running 965P chipsets, this compatible RAM List was created by a Gigabyte Technician. These modules are only tested on the boards specified (Gigabyte DS3, DS4 and DQ6), but they should work on any 965P board. This Xtremesystems DS3, DS4 and DQ6 (965P Chipset) Memory Thread can be of interest as well. Kemo6600 posted a list of confirmed Micron D9 Memory Modules over at XS that will help many people out who look for good Overclocking memory.
Comments appreciated, more to come. Any errors, additives or whatever, please point them out, I am SURE I have overlooked something in these 6 hours of research and typing.