|Benchtest it 1st!|
It's stupid to just go slapping it in the case without testing it. It's easier to install the CPU, heatsink/fan & RAM before installing the board anyway, so why not take the few extra steps to benchtest it? That way you'll know for sure that it works. And if it doesn't work once it's installed in the case, you'll know YOU did something wrong during the installation.
"I replaced the power supply, no power up"
What did you replace it with? Was it a new unit or used? What are the specs? Did you test it before installing it?