I just finished building a computer (well I guess "finished" is going a bit far considering) and I'm getting no monitor signal. The parts I'm using are:
CPU: AMD Phenom 9850 Black
Video Adapter: ASUS EAH3870X2
Mobo: Jetway HA04-EXTREME AM2+
RAM: 4X Corsair 2GB DIMMs
I know the monitor works on other computers and on staring up I'm getting the one beep POST code for no errors. I've tried another monitor with no luck, as well as another cable. The mobo has no onboard video adapter so I have to go through the graphics card. I've also tried disconnecting the hard discs and stripping it down to one DIMM, and switching the video card from one PCIE slot to the other. Anyway thanks in advance for the help.

POST - certifies (?) the health of the MOBO
Monitor/cable - works on another computer

At this early stage in the build process, with nothing else to go on, I'd be immediately suspecting the video card itself.
Do you have another you can try temporarily or take out of a nearby computer?
When the monitor is ON but not plugged into the PC, do you get a "no signal" message onscreen (good if yes)? When you then plug it into your new PC does the message go away even though the screen does not come up?
Have you checked the BIOS to make sure nothing in there is inhibiting graphics? (grasping at straws here)