Rack server cpu upgrade: how to find out what's compatible?

I have a few 1u rack mount servers sitting around that are running nicely but have older CPUs (eg. AMD Opteron 252's). I was thinking of throwing some new CPUs and extra memory in to get a performance boost.

What's a good way to figure out what CPUs (if any) are compatible?

[Update] One I was able to dig up is: HP DL145 2x 2.6GHZ OPTERON 2GB MEM 80GB SATA (dual opteron 252).

From serverfault Parand

• Start by telling us the make and model of the server...

  If you currently have 252s, the 254 and 256 will almost certainly work. They're the same proc, with a slightly higher clock. It's possible your server's MB will not recognize these processors however.

  Dual core socket 940 Operons exist, like the 285 with the same clock you have now, and twice as many cores. But your server might not accept dual core processors. The 290 is also available with a slightly higher clock and dual cores.

  Parand : The problem is the servers are sitting in a colo and I don't know their make and model. I may be able to dig it up, but I was hoping something like lshw would reveal enough to figure out if an upgrade is possible/desirable. Is there info from lshw or another command that would be helpful?

  Parand : Ok, found docs on at least one of them: HP DL145 2x 2.6GHZ OPTERON 2GB MEM 80GB SATA .

  Chris S : That particular one is compatible with the 275 and 285 Dual-Core Opterons. Also the 246, 248, and 252 Single-Core (which already have the fastest of). I'm not sure what info comes up in `lshw`, so I can't help with that one. If you can get into iLO it will tell you the exact models.

  Parand : Thanks for the detailed and excellent responses Chris. Looks like the 285 upgrade might be worth it, it should double my cpu performance.
  From Chris S