Ok,first thing is;
When you removed the cpu did you clean off the thermal paste/compound and reapply new before replacing the cpu ?
The 24 pin atx power supply is normally 20+4,depending on the power required will determine if you need the +4,but i think you are ok with the 20.
Your psu will be ok for now but i would recommend you get something in the region of 500watt and branded.
You really need to remove the dvd and cd drives for now and just connect the harddrive and one stick of ram and alternate between the two sticks of ram.
If the motherboard has onboard graphics,remove the graphics card and try again-the bios should default to the onboard graphics if indeed the motherboard has this.Sometimes you may need to enter the bios to change the graphics configuration.
It is possible the motherboard is damaged.