Dell 0D883F CPU upgrade options?

I have a Dell T5500 with a Intel Xeon E5504 CPU and a Dell 0D883F motherboard.  It already has a SSD and the RAM has been upgrades to 12GB but I'm looking to upgrade the CPU.

I know the machine is pretty old but it doesn't run too badly.  I'm under the impression the mobo is specific to this model and can't be swapped for a standard home board so does anyone know which (if any) CPU would run on it?