I'd wait a little bit before buying anything. The next generation of Opteron CPUs will bring a lot of changes, the most significant being, IMHO, DDR2 support and Pacifica virtualization. (I know DDR2 doesn't mean much performance-wise, but it'd suck to have to buy all new RAM.)
Those dual core Opterons are tempting though.... but I run VMs so I'm just gonna wait! 🙂