Can i overclock NB without a multiplier? (phenom II)

So i think i have reached my limit on overclocking my Phenom II 955 C3 @ 4.0Ghz due to my NB. I would love to try and raise my NB multiplier but... i don't have one... These are the NB options i have.

CPU/NB Frequency
CPU/NB Voltage

and then

NB Voltage
NB 1.8V Voltage
which i read to leave these 2 on auto.

I figure i just raise my CPU/NB freq to what i need but what about voltages? should i use the same chart for multipliers/voltages as i would with my CPU/NB freq/voltage? CPU @4.0Ghz = 2539MHz/1.25v NB? (cpu freq x 2 = ?? / 3.15 = target NB freq)

Another thing... At 4.0 Ghz my computer can take about 1 1/2 hours of prime95 blend before bsod. But i have NEVER had a bsod or overclocking error when i didn't expect it (playing games or benchmarking). and i have had my computer running at 4.0Ghz for about a year. Is there something else that gives you a more realistic test for overclocking? because i believe most wouldn't consider a 1 1/2 hour prime95 test stable, but yet it is.

Edit:
To help out with my question, i figured i should give some more info. I have the ASUS M4A79XTD EVO AM3 with a 790X NB chip.

And here's a pic of my bios settings. Ram is at stock (9-9-9-24-2T)

I'd monitor the temperatures of the CPU. If I were to guess, I'd say that the bsod happens because the CPU overheats after ~1.5 hours of being run at full load. Many games these days are running at far less than 100%, so the CPU likely doesn't get as hot.

http://www.tomshardware.com/forum/ [...] ps-voltage
has a few sample voltages

Is it a 955 or 955BE? Also, what mobo are you using?

From http://www.overclockers.com/forums [...] p?t=629758

that guy's running at 4GHz
Core speed: 4000 Mhz
Multi: 20x
Sus Speed: 200 Mhz
Vcore: 1.4225v

CPU-NB Freq: 2600 Mhz
CPU-NB Volt: 1.225v

HT Link: 2000 Mhz

Ram: DDR3 4096 MB
DRam Frequency: 800 MHz 9-9-9-24-33-1T
Voltage: 1.6v

Temps:
Idle: 31C
Load: 52C

Although granted he is using a core multiplier rather than NB freq modification. I'd try mimicing the rest of his settings and seeing how it runs.