Don't mean to butt in but I have the same mobo as you do Road_Star. Might wanna turn AI Overclocking Tuner off and set it to Manual. You will start off at the default fsb which is 200Mhz. From here, you can bump it up however high you wish to go.
You have the DRAM Frequency next. I am running my memory @ 1:1 ratio which is the 400 setting. I have been just as successful @ 5:4 as well, this is the 320 setting. Using the 320 setting will keep your memory @ 400Mhz (Using DDR400) at a FSB of 250. Using a 1:1 will result in a 500Mhz (Using DDR400).
I'm hitting 246Mhz FSB with a 1:1 ratio/relaxed memory timings (3-4-4-6) on a 2.8C. This results to a 3.42Ghz overclock running the memory at a PC-3900 rating.
In the same area, you will see the option to adjust the VCore Voltage. I've read that 1.575v is acceptable for a EE, though 1.6v may require a better HSF.
I set my DDR Reference Voltage to 2.85v just because... lol. Performance Mode is set to Turbo on mine as well.
Under Advanced>Chipset, you will be able to adjust the memory timmings here. To do this, change the Configure DRAM Timing by SPD to Manual. My memory is pretty weak, so I had to set it to 3-4-4-6.
I turned Acceleration Mode on Auto. This was according to X-bit Labs results when testing the i865 vs. i875 way back when.
I also turned off Spread Spectrum, according to various articles, this hinders maximum overclocks.
Few things to keep in mind. If it will not stay stable in when loading Windows and should reboot, try lowering the FSB down some. If it is stable, try lowering the memory timings.
If it doesn't boot up period and you get a black screen, don't panic. Just unplug the machine and reset the CMOS. Should be on page 2-20 of the manual.
I envy them Extreme Editions... lol. Have fun with the new machine.