I'm building a new pc with this Gigabyte motherboard(GA-

MA790FX-DQ6). My spec :
AMD Phenom 9850 Black Edition Quad-Core 2.5GHz
THERMALTAKE W0132RU 1000W MODULARIZED ATX CROSSFIRE ATI
New Corsair Dominator 4GB (2GBx2) DDR2 1066 with Fan
Visiontek HD 3870 X2 Oc Edition
Western Digital Raptor® X 150GB/16MB Cache/10,000RPM SATA
Thermaltake water cooler bigwater 760i

The motherboard doesn't power up to POST screen!! It's a new
pc. When i turn on the power, cpu is working and
everythingelse but Post screen doesn't comes up. Nothing
comes up on my monitor. I took out the ram and there is no
beep sound from the motherboard! Do i have to reset the
Bios? I also want to know if this graphics card(Visiontek
Radeon HD 3870 X2 Overclocked Edition)compatible with this
GA-MA790FX-DQ6! Also on the mobo's Chassis intrusion header
doesn't have a cap on? Does it suppose to have a cap on? Is
it the 4Gb ram(New Corsair Dominator 4GB (2GBx2) DDR2 1066
with Fan
) that i have cause the motherboard not POSTing? Does the
all the fan headers(cpu_fan,sys_fan1,sys_fan2,power_fan)
have to be plugged in?
Well I thought it was problem with graphic card so i tried
it with another card which is Radeon HD3650 512MB DDR2.
Doesn't work either! Nothing comes up on my monitor not even
Power-On Self-Test (POST) There isn't any beep sound either!
Do i have to reset the Bios?

Need help please!

Clearing cmos would be the 1st thing I do cuz the previous user might have used conflicting bios settings.

Clear cmos, try each stick of ram in each of the slots, hook up ANY fan to the cpu fan connector or else the mobo may refuse to post due to no fan present. By default, a mobo will check for cpu fan and if it can't find any, it won't post. You should've build the pc with a fan & disable the cpu fan detection in bios in advance of moving to water. If you did all of above and still nothing, it might be your h2o loop.