In mid-2001 I had to fly from San Jose to Washington Dulles with a Sun Netra 1120 server. It's 4U and FULL length. Weighed about 60 pounds.
I was late for my flight and the datacenter I retrieved it from had no packaging for it so I just carried it onto the plane - bare.
It fit in the overhead.
Long shot, but I'm currently in a position to highly appreciate some new hardware at the moment.
Ultimately I think my answer would be weighing the machine's rareness or unusualness against its power draw, fan noise or other annoyances. More of the former means that I'll accept more of the latter :) but in a worst-case scenario I'll just collect stuff and file it away, for example I was given some Sun V240s a while ago that I hope to be able to fire up when I can rack them in a well-ventilated faraway corner where I can't hear them.
Practically speaking I'm curious where people get rid of i3 based systems and multi-TB HDDs at the moment; I (crazy badly) need a new file server, and (for obscure, fun reasons) saving up for new HDDs is taking quite a few more months (and proving more stressful to my family, sadly) than I'd anticipated.
I just replaced a mirrored pair of USB drives running off a Mac Mini with a NAS from Synology. File server, plus Plex server, plus backup server, all in one. It won't be cheap (even factoring in the Australian duty) but it's something that can be tucked into a closet out of sight.
I've long observed Synology's tech and thought it pretty cool, but in this case I'm trying to consolidate as much as possible onto one machine. I'm trying to find something i3-compatible which has a few PCI slots (ahahaha) - if I can, I can toss one of the old Creative X-Fis I have here in it and get amazing sound straight off the machine with the disks in it, and I can also use some of the other PCI cards I have here too (my pile of junk doesn't have any PCI-E stuff in it yet).
My main focus is just saving up for the HDDs right now. My goal is 5 5TB or 8TB disks so I can use raidz5, which offers 60% storage efficiency and the ability to lose any two disks without issue. The 18-24TB of space is actually needed right now in order to do some serious dedupe across the literally dozens of HDDs I've collected over the years - I've been needing a consolidated file server for about a decade, things have finally snowballed to the point where some of the disks are clicking and I haven't had access to my data for about a year now, heh.
PS - Australia's Internet situation is basically a gigantic laugh (I can only get ADSL2+ - 80KB/s upload :D - where I live), or I'd have pushed everything over to a friend in the States who's letting me borrow his ZFS pool until I get my own set up.