Looks like the Opteron 185 launched in March 2006. I don't have any experience with it but all the posts I saw seemed to think well of it. A number of posts commented that it was very similar to an FX-60 but with a locked multiplier and aimed at 24/7 server business. Seems to be around the -5000 level of performance; not sure what CPU you're running now.
It apparently sold for a bit over $1000 originally, hope the price has come down :D
I still have one running in my Asus A8V-Deluxe, with 4GB of registered ECC DDR400. No problems with the chip itself, although the Asus BIOS didn't recognize it and therefore didn't enable C'n'Q for it. But I copied some ACPI tables from an Opteron 280 and tweaked them to get that enabled. It's actually still running OpenSUSE 10.2 with cherrypicked patches from 10.3; I've been loathe to update it because it's basically just humming along. Looking at my logs, "last reboot" was November 17 2007.
It was around the $300 mark when I got it, replacing an X2 3000+ which I gave to a friend.