No sure about 64gb, but for performance/watt and reliable Linux I can really recommend the Amd p16s and t16(s?) machines from Lenovo. Have about seven in the office and they are excellent.
I too, as someone in devops, am wondering what you need that much memory for. Do you simply really like VMs? :)
Also, have you considered doing the really heavy stuff remotely? Whenever I need desktop type power (16 physical cores and 128gb memory) I simply wake the desktop, ssh into it and do it there.