Re: Dumb Question: Why not more RAM ?
"So why not just put a shed load of normal DRAM memory into the machine, and let the O/S filesystem caching kick in ?"
Because having your cache reads and writes on the same memory bus doubles the traffic, effectively halving the data throughput.
As PCIe is on the memory bus anyway, you write once to PCIe. You presumably read it off the card without involving the system bus at all.
That should, in theory, prevent the system bus becoming a bottleneck....