My guess would be keep tinkering in bios until you get it otherwise you said you flashed it and it's newest.
Try defaults/turbo settings.
I doubt it's cpu.
I doubt it's ram.
I don't they'd ship a CPU that couldn't break 600 mhz. :?
I'm guessing you don't want to/can't swap cpu's from another computer real quickly.
I'd say if you can't get it working it's probably a messed mobo.
I'd mess with it a bit more before I tried to deal with returning since you probably bought it online.
Sure you didn't leave plastic on the processor or something stupid?
Perhaps try reseeding HSF although you haven't gotten any heat issues so I still say mobo.