A really huge dutch community has recently switched to Dual Opterons for their backend (they've got a Dual Opteron 242 and a Dual Opteron 240). Both are database servers only. They've replaced them for Dual MP 1600+'s, although the processor speed hasn't upgraded, the load went down with 66%.
Needless to say that they are very happy with the servers
On the other hand, they've got 4x Dual Xeons clustered for the html work though ;)