Minimum Requirements for Combined Domain Controller Aspect Server
AFAIK, the DC and AC 800M OPC server roles have very small requirements on CPU and RAM.
I can not answer for the OPC servers you did not name.
An aspect server enjoys a fast disk with write cache
Typically you are better off spending money on fast CPU (GHz) in clients. An aspect server will work with a slower CPU.
A connectivity server may need some extra disk for history logging (Basic History drain 24 bytes per sample and log)
V6.x: 8 GB RAM. 2x-4x processor cores at 2.4 GHz. RAID disk with write cache.