Dual cpus for PE 2900

Hello All, I am trying to find out what is the fastest cpu that can go into my PE 2900. I purchased this on the cheap from a church ($300) never been used still in box. I am sure I'll have to do BIOS updates and possibly replace some internal batteries, but can someone tell me the fastest processor that this unit can take. It presently has a single PROCESSOR, 80563Q, XCL, E5345, LGA771, G0 Thanks ajinc1