I'd say sell your old computers, and get a monitor to use with the macbook air, but don't buy the monitor til you've used the air as is for a month or so. I think you'll find even the 11" display is great for what most people do.
I can see it being useful to have a higher res screen and more cpu power for movie and music production. The ipad is ALMOST ready to replace a laptop for onstage virtual instruments, etc. I'd love the A6 in a non-retina one, personally, since I don't care about resolution that high, generally.