There are tons of PC game collectors, with communities scatter throughout the web (do some searching).
The only real 'holy grail' PC games are ones that created a genre or changed gaming forever, and we are talking original releases here. Things like Akalabeth and Colossal Cave Adventure. However these are super old and 'rare' doesnt even begin to describe it.
Since anyone can make any game they want and release it on PC (in any quantity), there are probably thousands upon thousands of obscure "rare" games you can find, yet they aren't worth diddly squat.
What may mean a lot to one collector, another won't even keep for free. I am sure those Japanese Neverhood versions are sought after by Neverhood fans but it isn't like having a Nintendo World Championship cart, where it is highly sought after by every serious NES collector. This may be actually why PC collecting scares off so many people. A game on auction today may fetch $400 because three interested parties noticed it, a month later it may sell for $50 because only one has seen it. Only really popular franchises (like Ultima) are always monitored and sought after by many many people.
Because of the [relative] lack of organization of PC releases, it is difficult to assign any kind of value to the truly rare and obscure stuff.
PC collectors tend to collect either stuff that has a personal meaning to them (favorite genres or series or nostalgic games from their childhood), or revolutionary milestone games (like the two I first mentioned).