Maurice - 11:44 am on Apr 6, 2011 (gmt 0)
this is how we did it for the BT Worldwide extranet back in the day.
1 Dev system
2 Test System (run as a technical copy)
We had identical hardware for all 3 systems (3x 4u sun boxes) - the main networks/tech guy joked he wanted all the parts from the same production batch.
We would have liked a fourth machine as a hot spare in the exchange as to replace a hardware failure we would have had to fly the test machine up. And I suspect we might have had to threaten BA or Easy Jet with being black listed if they refused the server a ticket.
We could test then upgrade the live by swapping drives from test to live (after changing a few config lines to change the domain mostly)
Do you really need hot swap its going to be expensive and you do need a separate machine just for testing before going down hot swap raid arrays route.