Hey guys,
I have also this motherboard. My plan was simple. DX48BT2, Q9450 (2.66), OCZ (1,9V, 7-6-6-20 default), get it to 1600 FSB+3,2Ghz CPU and I hoped it will worked like a charm :-) Well, not really. This mobo really needs love and a lot of playing with the settings.

I was able in about half hour to put it on 1600 FSB by these two steps.
1) CPU overide
FSB 400 mhz
Multi 6 (2.4 CPU underclocked)
C1E disabled
IESST disabled
Mem overide
REF 400
MEM 1333 (memory not overclocked)
vMEM 1,54 (default)

2)Mem overide
REF 400
MEM 1600
vMEM 1,82
Other settings+voltages default.

With these two steps I was able to put mobo to 1600. If I set all walues at once mobo not post. By these two steps it post, also acts stable enought to boot Windows, but not pass memtest :-(
I tried nearly everything. Changing memory voltages higher, loosing timigs to 8-8-8-24. But I still get so many erros in mem test, about 30000.

I returned this memory to manufacer, I get new pair (1,8V, 7-7-6-20) -> same error. Well, it must be definitively by hands!!!

I started experimenting with voltages and discovered that pusshing vMCH 1,25 -> 1,35V will eliminate a lot of errors and also big continuous error. After that there were 10 or 20 errors per pass, so much less. I also tryied vMCH 1,45V and vFSB 1.3V not helped. Much more helped experimenting with advanded timing. Very good result was 7-7-6-20|54-5-10-6-6. But I allways get mem error 0002b12336c 689,1MB. Always the same error more times. I nearly went again to shop to return this pair.

LUCKYLY I found comment from TONY on OCZ forum to try to change 400/1600 to 333/1333. I already noticed that it gives mem at same speed 1600, so it looked useless to me.
IT HELPED A LOT! (Memtest passes at default voltages.)

Right now I can pass memtest and running Windows somehow stable. (Mobo often not post after hard power off, also ramdom reboots)

QUESTION: Do you guys know difference between settings 400/1600 or 333/1333? Is the speed same?

What I noticed more, is that THERE IS NO VOLTAGE THAT YOU SET! This really shocked me. It can be by power suply. My PSU is Enermax 625W. I thought it is enough, but now I'll definitivaly buy new 1000W at least.

Without enabled Enhanced Power Slope(EPS) vCPU is set to 1,325V, but in reality I had 1,18V. In high load it also 0,02 goes down. With EPS there is 1,25 deaful CPU voltage. But it is still not 1,325V. Same situation with vMCH. Right now I have set it to 1,4, but in fact here is 1,31.
QUESTION: Is it mobo, PSU, Intel tring to protect components, or beta Control Center? Can you please some of you install iDesktop Center and check the vaules it if is not only my issue?

Right now I'm nearly stable with this settings:
vCPU 1,325
EPS enabled
C1E disabled
EIST disabled
multi 6x (2,4 CPU undeclocked)
FSB 400
vFSB 1,3
vMCH 1,4 (I discovered that increasing vMCH reduces reboots. On default settings it restart in about 10 minutes in Everest stress test. With higher Everest run 6 hours no restart, also pass memtest 2x)

REF 333
MEM 1333
7-7-6-20|72-5-10-6-6
vMEM 1,82V
CL Auto (1T)

I'm still having reboots in after about 6-10 hours. I can run mem test whole time, Everest stres test, no problems. System is stable, but while playing Winamp 8 hours it reboots, it is really weird. I think it is not Graphic card as I previously read. I run GPU benchamrks many times. Also ATI tool scan for artefacts, no problems.

QUESTION: Can some of you guys please post default values for Core 2 Extreme at 400FSB also with MEM settings?

If you guys will have some ideas please write. This forum really helped me. This mobo is like a woman... First you must compltely understeand it, then it allows you to touch the beauty :-)