It's high time for Intel to find a proper response, as the competition is taking the wind out of Intel's server CPU sails. What's the answer? A Xeon based on the Core architecture: Woodcrest. We compared the Core and K8 architectures just a month ago. Memory disambiguation, large OOO buffers and a large but low latency shared L2 cache should make the Core architecture more efficient in server related tasks than any other x86 CPU.Check it out over here.
Intel Woodcrest vs AMD Opteron vs Sun UltraSparc T1

AnandTech compared the newest server processors from AMD, Intel and Sun: