Should I buy a Dual Opteron or Dual Xeon for use as a MySQL Database Server?  

Actually the Opteron is a much better processor. It is 64 bit and so if you are running Linux or Windows 64 than you can take advantage of it. Databases can really take advantage of 64 bit chips. The 64 bit intel chip is called Itanium. You should be comparing Opteron versus Itanium and tell you the truth Opteron is better than even the Itanium!

Look at HP or IBM for Opteron servers. Dell is still selling Intel only.

" The comparison we've made here is a very important one; it identifies Intel's strengths and their weaknesses with Xeon, and it crowns Opteron a clear multiprocessor winner. An area that we didn't touch on is cost, which is where AMD truly shines. The Opteron 848 processors we tested are around 1/2 the price of Intel's 2MB L3 Xeon MPs and we have not seen retail data on how expensive the 4MB parts will be.

In a 4-way configuration AMD's Opteron cannot be beat, and thus it is our choice for the basis for our new Forums database server. We'll be documenting that upgrade in a separate article so stay tuned."
http://www.anandtech.com/it/showdoc.aspx.

The link that the previous poster proves it. Smaller is BETTER in this case because it is an indication of the amount of time

n a 4-way configuration AMD's Opteron cannot be beat, and thus it is our choice for the basis for our new Forums database server. We'll be documenting that upgrade in a separate article so stay tuned."
http://www.anandtech.com/it/showdoc.aspx.