Ok, thanks to workplace, I have an Intel Core2 Duo E6850 for testing, on my D975XBX2 (rev 504, BIOS 2792) motherboard.
I previously had an E6600 ( 333 FSB, CPU overclocked to 3.0 GHz) in the motherboard, and 2 x 2GB Infineon DDR2-667 ECC DIMMs (4GB total). I swapped CPUs without bothering to clear CMOS. With the new E6850 in the motherboard, it booted successfully!:)
...but now for the "Intel BIOS hall of shame"

After my first successful boot, I decided to reset all my CPU/mem settings from manual to 'auto' (i.e., restore those settings to DEFAULT.) After doing so, system hangs at POST, doh! Fortunately, I moved the board jumper to "maintenance mode", and I was able to get into the BIOS-menu again, change those settings to MANUAL (333 REFERENCE FREQ, 667 MEMORY FREQ, CPU-freq = auto. Everything else, i.e. voltages, are at default.) With these settings, I once again can boot normally.
When I had the E6600 and 4 DIMMs in the motherboard, my D975XBX2 refused to POST @ REF-FREQ = 333MHz. If I took out 2 DIMMs, or set REF-FREQ = 266 (and MEM-FREQ=533, to ensure proper 1:1 mem/fsb operation), then the board would boot fine.
<update>
Well, I'm happy to report that I've just tried 4 DIMMs ( 2x 2GB Infineon DDR2/667 ECC, 2 x 2 Crucial 1GB DDR2/667 ECC) at BIOS-settings of ref-freq=266, mem-freq=533, cpu = 334 MHz. System POSTED fine into Windows XP/Pro (only 3.5GB is usable under the 32-bit O/S, of course! but CPUz 1.4 reports all 6144MB.) Yay, it works!
Note, I didn't try 4 DIMMs @ ref-req=333 (mem-freq=667). I suspect I'd get the stupid HANG. Since I already have 4 DIMMs working @ ref-freq=266 (mem-freq=533), I don't really feel like opening up the case again just to test this theory.
Someone else posted a report where his E6850 wouldn't even POST on a pair of DDR2/800 (non-ECC) RAM. He had an older pair of DDR2/667 DIMMs, and he was able to POST fine. I'm wondering if he had to manually set/override the reference-FSB and memory-FSB (like I did.) Could someone with DDR2/800 RAM try some experiments?
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To recap so far:
Intel Core 2 Duo E6850 on Intel DBOX975XBX2 (rev 504, BIOS 2792)
CPU is *NOT* overclocked -- my target is a production server, so I will run at stock (3.0GHz) speed.
In the BIOS, the memory-freq and REF-FREQ settings must be MANUALLY set. Leaving them on AUTO caused my D975XBX2 to fail POST.
If I set the CPU-freq=DEFAULT, Reference-FSB=333, and MEM-FREQ=667: system boots fine with 2 (DDR2-667 ECC) DIMMs. (I did not test 4 DIMMs.) CPUZ 1.4 confirms 1:1 operation FSB/MEM relationship.
If I set the CPU-freq=DEFAULT, Reference-FSB=266, and MEM-FREQ=533: system boots fine with 2 (DDR2-667 ECC) DIMMs. However, now, the motherboard boots up at E6600 speed (2.40GHz), instead of the proper 3.0GHz. It appears the buggy BIOS uses the Reference-FSB doesn't auto-set the CPU-freq properly.
If I set the CPU-freq=334 (manual), Reference-FSB=266, and MEM-FREQ=533: system boots fine with 2 or 4 (DDR2-667 ECC) DIMMs. CPUz/Windows reports 3.0GHz when CPU is under load.