Before I'd write off the mobo, I'd strip the machine down to the bare minium-power supply, one stick of ram, take out the graphics card (if you have a vga output on the mobo), unplug any/all CD/DVD drives, remove the sound card (if you have one)-and try to boot up. If it fails, then take out the stick of ram and replace it with the other stick (single sticks should be in the right most slot).
If this fails, I'd double check all the connections and wiggle cables to rule them out. If nothing helps, then your mobo might just be fried
Of course, if it's under warranty, tell Dell and they should replace it.