How far can I OC my GTX 580?

Hey!

I'm a bit curious as to how far I can overclock my current GPU, it's the ASUS GTX 580 DirectCU II(Link: http://www.asus.com/Graphics_Cards/NVIDIA_Series/ENGTX5...)

System:

ASUS P8P67 WS Revolution
i5-2500k @ 4.3 GHz
ASUS GTX 580 DirectCU II 1.5GB
Kingston HyperX 1333 MHz 2x 4GB
OCZ Agility 3 120GB SSD
WD Caviar Black 1GB 7200RPM
Corsair 750W 750TX

Currently I'm using the ASUS GPU Tweak software to set my OC on every time I play a demaning game such as Battlefield 3.

Default values:

Core Clock(MHz): 782
GPU Voltage(mV): 1050
Memory Clock(MHz): 4008

Overclock Values:

Core Clock(MHz): 908
GPU Voltage(mV): 1138
Memory Clock(MHz): 4130

I'm using the Core Clock&Voltage Sync so they scale together, it has worked very well so far(not sure if it is recommend though?).
Currently with the OC on, it seems that the Voltage is reaching all the way to the end of the slider, I'm not sure if this means that it is the maximum Voltage the card can take or slight "glitch" in the program.

Thanks