Actually WP Card is a RAM.
But its driver run before anything else (like apcahe and other programs)
Wp-Cache plugin(s) can control to this RAM.
When there is a request from server (through network adapter), its driver begin to run and reply go to direct to client from ram-cache (by-pass hard disk, mysql etc..)
It can fly sites faster then YF-22 :)
If exist like this hardware, I can pay 50-80$
and I’ll proud of my server has hardware with WordPress stamp ;)