Better silicon and usually a different socket. Often uses FB-DIMMs (
). Some server chips are made for desktop sockets and normal RAM as well.
So they use the best silicon for best reliability, right? Doesn't the new nehalem xeon use the LGA1366 just like the desktop i7s? I know that the opterons use socket F which is opteron-only.
So basically there's no point in using a server chip, seeing as you can get desktop chips that use FB-DIMM (can't you?) if you really need that.
Server chips sound like the FireGL/Quadro of the CPU market. Not much point in spending the cash on them.