Just my observations on what's collectible in the PC world and what's not.....
Currently late 486 and early Pentium stuff seems to be getting extremely popular on the DOS PC end of things. Though I'm seeing more and more "retro rigs" being made out of PII-P4 era hardware now. I was going to junk my Dell Dimension 4600 - now I'm holding onto it (esp since I have all the original docs/discs/drivers with it). I think some of it though is because those systems are not exactly "vintage" yet though so they can still be had cheap. The only way a 486 or early Pentium is still cheap is if you are like me, already have one (I have both actually) and are just looking for parts and buying them new from china (ie Cache RAM, SIMMS, etc...).
386 era is losing popularity, 286 seems to be still getting some attention. I see a couple of people on Vogons building out 286 systems this year. Seems the interest in them is their lack of commonality more than anything, and it seems what gets assembled from clone parts today for a 286 is usually a fairly modern ATX or AT tower case with older guts in it and some 486 era parts to make it work.
8088/8086/XT/PC era is just too out of reach for some people now due to the cost of those systems, probably CRT monitors being a big problem because whose going to pay $450 to some I.C. hardware provider to buy a 14" EGA LCD panel they will only use with a small section of early eighties PC clones?
As always, the big names (IBM, PC's Limited/Dell, Compaq, and so on) are popular as usual for historical purposes.
One thing, through me collecting all this stuff, it seems PC's go through these phases - unlike Hobbyist computers of the 70's which ALWAYS required some technical knowledge and a soldering iron to keep going.....
1.) PC is new, costs a ton of money
2.) PC is a few years old, not worth as much, but still usable to comfortable to use
3.) PC is getting old, replaced by new one, kept in closet/attic/basement
4.) PC Is really old, given to kids, useless, kids break it, gets donated, trashed, etc....
5.) PC is totally junk in everyone's eyes, this is where the lot of them get junked/scrapped or destroyed, the rest go to nostalgic old gits like me who buy them cheap (like I did 80486 in 2001)
6.) PC's original audience of it's generation gets older, gets nostalgic, a few start buying them cheap
7.) PC's sellers get wise and notice they could push prices up,. prices go up
8.) Noticeable holes in the PC's current needs either based on old hardware (or not) lead to many indie maker-types to make replacement hardware (I'm talking XT-IDE, that 3D Printed Drive tray a few threads below, the IBM PC 5150 motherboard kit from that little scientific company, etc)
9.) It gets en vogue (for as in vogue an old computer can become) because those new technologies above make it easier to get into those machines, and it negates the expensive risk (like in the case of XT-IDE and solid state drive on modules vs. buying 30+ year old hard disk drives that may or may not work, and their associated host adapters).
10.) Machines officially become vintage, supplies are dwindling, so people without the inclination/repair skills to go deeper than a screwdriver and error codes leave said hardware to those willing to don a soldering iron and test equipment and save a poor dying piece of hardware from disappearing by actually fixing it from the board level
11.) Machines become very scarce, prices get insane, investor types sell their stuff off for $1K+, those who love it or have parts mutts keep going with their new ever-growing knowledg