The Intel Xeon E5450 is an Intel LGA 771 socket CPU.

The Core 2 Quad Q9650 is an Intel LGA 775 socket CPU.

The LGA 771 Xeon CPU you are interested in cannot be used on a LGA 775 motherboard. If you attempted to install a LGA 771 product into a LGA 775 motherboard you would damage both the motherboard and the CPU in the process.
For example the HP Compaq dx2400 Business Desktop PC says it supports
Core 2 Quad but no Xeon support is mentioned.
This is because the dx2400 officially only supports LGA 775 CPUs. HP isn't going to list LGA 771 CPUs that require a hardware modification in order to support. There is no way to gurantee or predict if a motherboard will or will not accept the CPU after the LGA 771 to LGA 775 modification. The adaptor LGA 771 to LGA 775 modification is a "do at your own risk" type modification.