We have clustered dual opterons as welll as some single and dual cpu desktop/workstation machines... we like them. We don't as yeah have any running oracle although we do have 4-way xeon mp's doing that.
FWIW, I have a FC2 dual opteron workstation, and just in the last few days got Oracle 9i up and running on it. Not a production server, mind you, just my own sandbox.
I've found my workstation to be (subjectively) *very* fast. I find that WinXP inside a VMWare host is nice and responsive, as is Lotus Notes under wine (CrossOver Office). In a slightly more quantitative test, I can compile Postgres from a clean source tree in 1.5 minutes using "make -j 2", versus anywhere from 4.5 to 12 minutes on other machines that I have readily available (part of that is undoubtedly scsi vs ide drives).
Joe
