In a post above you wrote "And still if i plug my monitor on onboard graphics it stills runs fine..dvd boots and bios function load.." Based on that statement, you should be able to configure Init Display First to PEG (PCI Express). When the GTS250 is inserted, the BIOS doesn't know that it should use it.
I presume that you read this in the manual?
Dual Display Configurations for the Onboard Graphics:
This motherboard provides three video output ports: D-Sub, DVI-D, and HDMI. Dual monitor confgurations
are supported in operating system environment only, but not during the BIOS Setup or POST process.