Memory usually doesn't prevent a system from powering up. What do you mean by no power at all? Do you at least get standby power? Are you absolutely sure that the power supply is good? Have you tested it in another system? Disconnect everything and keep the minimum components (CPU and only one memory module). You didn't list what video card you use, but it has to be a low-power card with a 350W PSU. Did it power up when tested outside of the case (presuming that you did that before inserting everything in the case)?