Here is the latest update. The progress has been a bit slow due to personal reasons, but it is clear that this chip has a lot to offer - I observed so many interesting behaviors and as others have commented, it is no doubt I will spend many quality hours to come.
Since there are many mysteries that I haven't even begin to explore, I think I will make a separate thread in the future with more details. The way NB behaves, the way clock generator works, and the way memory dividers work are all still big unknown to me. In this thread, I am going to concentrate on OC'ing results and brief impressions on its performance.
I have switched the HSF from Scythe Samurai to Scythe Infinity, and this seems to help for clocking quite a bit. At the least, I get more consistent OC results as well as slightly lower temperatures.
Thanks to Gikaseixas, I got more brave with voltages on NB. Previously I didn't give much voltages to NB because the default is 1.1V. In order to stablize the NB @2.6 GHz 1.35V was needed. For 2.4 GHz 1.175V suffices so it's quite a jump.
I have found a few stable combinations, with decent thermal characteristics suitable for 24/7 usage:
1) CPU 3.6 GHz (1.35V) | NB 2.6 GHz (1.35V) | 8GB DDR2-800/5-5-5 (1.9V)
2) CPU 3.8 GHz (1.39V) | NB 2.4 GHz (1.17V) | 8GB DDR2-800/5-5-5 (1.9V)
3) CPU 3.8 GHz (1.42V) | NB 2.6 GHz (1.35V) | 8GB DDR2-800/5-5-5 (1.9V)
3) is a little too hot for my taste and 1) performs better than 2), so 1) is my current favorite. (and I am biased towards 3.6 GHz

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3.9 GHz is not difficult for the chip itself, but at that point my air cooling is not cutting it. I can run some benches but not stability tests. From what I can see, it's simply a matter of temperature at that frequency. As has been noted, however, 4.0 GHz is a totally different story.
Also I have to compliment my board, despite its lacking BIOS for overclocking. Mimicking its P45 cousin, the board's usability is nearly flawless. Everything works as it should and even the SB750's performance exceeds my expectation. As a matter of fact, with 955BE @3.6~3.8 GHz the whole system feels extremely snappy and smooth. (This could be a halo/placebo effect of Windows 7, however)
Talking about recycling RAM, the whole transition didn't really cost me much thanks to my initial investment on NForce system a year ago. I even pocketed a few dollars after I sold my 780i board and purchased this 790X board. Can't complain.
