Thanks Blitz for the reply before.
My O/S is Win XP Pro, maybe this the problem.
On Device manager i found (?) string on Xeon(R) processor E3-1200 v2/3rd Gen Core processor PCI Express Root Port - 0151. This symptom like thread PCIe No Longer Works on Z68 Chipset After Upgrading to Ivy Bridge Processor
When you tried to disable VGA onboard, the signal to monitor is gone, completely disable (see manual):
18.104.22.168 Dual Video
The BIOS supports both single-video and dual-video modes. The dual-video mode is disabled
* In the single mode (dual monitor video = disabled), the on-board video controller is
disabled when an add-in video card is detected.
* In single mode, the onboard video controller is disabled when an add-in video card
is detected. Warning: System video is completely disabled if this option is disabled and an add-in video
adapter is not installed.
* In dual mode, the onboard video controller is enabled and is the primary video device.
The external video card is allocated resources and is considered the secondary video device.
thats why I'm just enable/disable dual display on BIOS. I will try enable Maximize Memory below 4G.