Thanks for the responses guys. That CPU comparison page is a great resource as well.
A bit of searching revealed that the Opteron 175 is effectively an Athlon 64 X2 4400+ Toledo, so I was able to see on the comparison page that the C2D really does have a significant performance advantage over the others.
Thanks for this post I was looking for a new server tonight and just picked up the new Xeon chip you guys spoke about from SoftLayer. My box was ready in about 1 and a half hours. We're just about ready to load test it! I'll let you know how it works.