Intel® Xeon Processor 3500 series
Intel Xeon Processor 5500 series
are what came with your computer, what one is in it currently?
That said, there really is not much of an upgrade path. Xeon processors are top of the line. Maybe an i7 would give better single core performance, but there are not a lot of processors designed for that socket.
Link
could you post what processor and graphics card you have so we have a better idea where to guide you?