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You will have to update the firmware to 1.3.5.x and the boot file to 1.3.5.06 before updating to 1.4.x, as stated in the release notes (available here).
It depends on how many devices you would like to run (and what type). They have a resource calculator here: http://virl.cisco.com/resource/
More RAM is definitely better, I just picked 16GB as a reference point as that is quite inexpensive nowadays...
I am not sure what your budget is, but another option would be VIRL: http://virl.cisco.com/
The benefit is that you would only need a server and a yearly subscription. For instance, with 16GB RAM and 4 cores, you can run 3 CSRs, 2 virtual ASAs and...
Even with those, it's not always possible to update without downtime. For certain upgrades, updating the ROMMON is necessary, and that takes down both supervisors.
Separate switches are the more guaranteed method.
