It's a tossup as to whether its the motherboard, an incompatibility or the PSU. My experience is the PSU is more often the issue, especially if it's an ATX model under 400w. If it's an incompatibility, a different brand of video card may work (AMD vs nVidia) but it's more likely an incompatible BIOS.
What happens when the monitor is plugged into the onboard video and the new card is in the slot? If it works, does the BIOS show any info or errors about video when booting or in the BIOS?