Before you go and buy new parts, we need to isolate the problem part which shouldn't take long.
First, can you post specs on your machine...CPU, RAM, PSU (400W or 500w?), Hard Drives, and OS?
Since one power supply (PSU) was replaced...was it for the same amount in Watts? Are you sure everything was plugged in properly...and was the power outlet on the wall you were using working?