What is the make and model of the power supply? There should be a description sheet that came with it.
What is the model number of the motherboard? It is usually printed on the board. Could be something like "Intel D945GCLG1". Might be a good idea to blow the dust off the board. Or do you still have the manual? Printed or on CD perhaps.
The 4-pin connector in the photo is like the one for the CPU.
Does one of the 4+4 connectors, in the lower left of your photo, fit? The connectors are "keyed" to certain connectors on the mother board so should be fine. Don't force anything.
When you reassemble don't forget to clean the heat paste off the CPU and heat sink and replace with fresh paste. Don't use too much. Just a "pea" size squirt will do. The force of the heat clamps will spread it around as you close it.
Let us know how it goes and don't hesitate to ask any more questions.