Just to add to QuadMaster, here's what makes opteron a server processor:
-Opteron has more than one Hyper transport link (3) so that more processors or more I/O devices can be connected to an opteron.
-Opterons can support upto 8 DIMMS so that the max supported mem capacity is 8GB per processor.
-Opterons use only registered memory modules which ensures that the memory is more reliable in storing & retrieving data since reliablity is highly sought after in servers.
-And ofcourse, the point that nulls any technical reason to buy: the cost of the opteron.
And yes, it is very much advisable to upgrade to A64 now, or if u can wait and have the funds, wait for dual cores.