FYI, I have a Pegasos2/G4 2B5 which is working reliable with 2GB of RAM.

My working DIMM modules are: KVR266X64C2/1G.

It was quite difficult to find DIMM which were working perfectly. Most of them seems to work but don't (rare random issues).
For instance, DIMM CT12864Z40B was almost working, it tooks me months to figure out that there was an issue. ^^