The E6400 Conroe core (if you found one of these it's pretty old) has an FSB freq of 266 MHz. Allowing for overclocking, the DDR3-1333 is probably at the sweet spot.
The E6300 or E6500 Wolfdale CPU's also have a 266 MHz FSB freq, but they run at 2.8 and 2.93 GHz, respectively. The much greater speed more than makes up for the smaller cache. And they are much better values than the Conroes.
Even on a G41 motherboard, you should be able to reach 333 MHz FSB. That will do two things: run DDR3-1333 RAM at its full rated speed and give you a core speed of around 3.5 GHz.
In fact, I have a system that I put together out of leftover parts and an E6500. It's running in a Gigabyte G41M-ES2L motherboard with 4 GB of DDR2-800 RAM. It's Prime95 stable at 3.63 GHz (333 MHz X 11).
Based on a sample of two G41 boards (yeah, big statistical sample
), you cannot push the G41 FSB higher than around 365 MHz.
They are obsolete but I love the Core2 chips.