What is the model of your motherboard Steinbert? Some Socket 775 motherboards will not support the Core 2 Quad Q9550 Processor. We need to know the motherboard model to know whether or not that processor will work with your mainboard.
Is the Core 2 Quad 9550 Processor you are looking at 95 Watt or 130 Watt? Is this system a custom built or a system that was pre-built by a company? What are your power supply specifications? You can find out the power supply specifications by opening up the side of your computer and looking at the power supply.
What is the cost of the processor you are looking at?