If your motherboard has onboard video, double check that it's disabled, or try hooking the monitor upto it to see there's any output from that (Happened to me once on a cruddy old PCChips board !)
Also, make sure the memory is seated properly. Sometimes you really have to push it hard and the motherboard sounds like it's gonna crack. I found the best way is to take the motherboard out of the case and lay it on a piece of thick cardboard (to stop the backside from scratching the desk) and push hard.
Finally, try clearing the BIOS settings, it's possible something got fscked up at the factory and some bad settings are stopping it booting.
