munnz - i'll jump in here and give you a simple answer, unlike some others.
The second processor must match the first processor exactly. Same speed, same cache, etc. Installation is simple. The CPU socket has a lock/release lever. Lift this lever to unlock the socket. Insert the CPU (take care to note the position of the key pin, and orient it to the key pin location on the socket). Fully seat the processor. Return the lever to the "lock" position to lock the processor in. Apply a small amount of heatsink compound to the top of the CPU, and install your heatsink. The Xeon 603/604 heatsink in this system uses (4) screws to attach it. Start the screws in opposite corners first, and then start the remaining two. Working in an "X" patter, begin to tighten the four screws, tightening the screws 2-3 turns and work your way back and forth until all are securely tightened. This is to ensure the heatsink seats properly. When the system is powered on, it will automatically detect the second processor.