Here is info for the Z620, and I don't know if this would work for the Z420. Both the buffered and the unbuffered memory HP has rated for these is ECC. The memory for the Version 2 Z600 and Z400 can also be either buffered or unbuffered, and also are all ECC. In all cases you are not supposed to mix types, and I've always gone for filling all memory slots with identical RAM sticks. I've gotten experienced at buying used HP memory off eBay to experiment with.
The top memory speed went up on the Z620 when compared to the Z600.... the Z620 can run the 12800 memory at full speed **if it has a processor also rated to that speed**. Otherwise the memory controller will default it back to 10600. With that in mind I thought I'd try an experiment.... fill my Z620 with identical ECC buffered 10600 memory from my Z600 stash of used eBay memory, and give it a try. Why? I found those memory sticks at low cost for some Z600 builds I was doing, and had extra.
Worked fine. Might be a source of cheap used HP memory for those of you students on a budget with a slower processor in a Z420 or Z620.