The only things you would have a problem with shipping back from a size and weight POV would be the case, PSU and possibly mobo. And screen, of course. Case & PSU could be bought as totally disposable items, then get a better one later.
Everything else would be really easy to strip out, re-pack and carry. And that's most of the high-value stuff - CPU, RAM & disks.
I agree with 1 disk for system, and other disks for data - if you can afford RAID then why not, although if cost is a concern (doesn't sound like it), then 1 big data disk in the PC and another for backup via network / USB is an option. And I agree about hardware RAID - SW RAID is dubious for several reasons.
If you have multiple disks, then you've lots of choice for dual-booting and setting up both OS's nicely.
As you say, for a server disk spindle layout is often important for performance, but for a home PC, it barely matters.
what cross fire is
Best avoided.
It's to link two (or more?) graphics cards together for ultimate performance, but generally is far more hassle than it's worth. See Keygrip's problems with SLI.
SD