I have an old HP ProLiant DL145 G3 server I'm now using mostly as a budget 3D rendering node. Right now it has two Opteron 2218 processors (each is dual core). I'm looking at picking up a couple cheap Opteron 2425 HE processors for it (hex core) to speed it up.
Both of these processors are Socket F. The TDP on the 2425 HE is lower, so I shouldn't need to worry about heat. However, HP only ever officially supported up to the Opteron 2220 for this model server, which is a "Santa Rosa" model processor instead of "Istanbul" as in the 2425.
My concerns would be:
I don't know if the OS starts up cores by itself or if it depends on the BIOS to enable them first - if the BIOS has to enable them, chances are it won't enable all six, since the system was only designed for dual cores (for instance the BIOS won't enable AMD-V even though the 2218s support it).
The 2218 uses a 1 GHz HT link whereas the 2425 uses 2.4 GHz HT. However, I've read that HT autonegotiates with a controller on the motherboard, so I'm guessing this isn't a problem but I'm not sure.
The 2218's voltage is listed as 1.30/1.35 .. I'm not sure which that is, but the 2425 is listed as 1.15V. I think the mobo negotiates the voltage with the CPU, but I'd hate to be wrong and have the new procs burn out.
(There's also the matter that the 2425's per-core cache is actually smaller (512k vs 1M,) which could have an impact on performance but I'd probably have to test them to see what works better.)
EDIT: I know off the bat that not all Socket F Opterons are interchangeable, since various models have different limits on the number of processors. In this case that is not a problem since both models considered are designed for dual processor.