If you take the board to your nearest computer shop they will test it for you, it might cost you a tenner or something but will be cheaper than buying a new m/b and if it is not the m/b at least you now know that. Or take it back to where you purchased it and get them to test it.
Go right back to basics and just have one stick of ram the hard drive and the floppy drive attached only....will it boot now... if not are there any jumpers on the motherboad...if so check they are set correctly....if its still nat booting can you change the RAM for another stick?
The PSU is a switched mode power supply, so will not start if it senses any fault on the mother board. The power switch pins connected to the power switch should be checked for continuity. New power switches do fail, double check all connectors. Some power connectors are a bad fit. If that fails get professional help.