I have a DS3 and have just solved a few problems by updating to F5. Sorry for your plight. I have nothing really to add from what you have already done except I have had some issues with SATA cables being very very flakey - a slight touch and no system. Is the psu okay, are the connectors to it okay? Really, though, it does not soiund like anything physical since you said it was running fine and you simply 'successfully' updated the bios. Sounds like something the new bios did/not do. Have you got spare ram you can try, maybe try different slots or just one slot? Take out all PCI/e cards but your video card. Unplug any extra hard drives, DVD writers, etc - prune the system to basic. And try again. If nothing happens, return the DS3.
BTW you did clear CMOS properly? You more than likely did, but just encase. Turn off and unplug the system from the mains. Wait 20 secs. Short the pins for 5-10 secs with a flat screwdriver (careful not to touch anything else). Start up again and hopefully Delete into bios.