You can get a boxed Fallout 1 here.
I don't say I so much collect PC games as I "accumulate" them. I pick up things that I might enjoy if the price is good and I won't likely see it ever again. And then I never get around to playing them.
Anyway, as I have said before, the really desirable PC games are some of the old adventure games, like Sierra's old bundle packs (King's Quest Collection, the Roberta Williams Anthology, the Quest for Glory Anthology, etc), Celtica, Chewy in Esc from F5, Gord@K, The Neverhood, and Alice: An Interactive Museum, to name a few. http://www.playinggamesinteractive.com can sell you some of these for the right price. An old Infocom game complete with all "feelies" might fetch a good price too.
As far as recent stuff goes, FFVIII is a bit uncommon, and maybe Sid Meier's Alien Crossfire. Abe's Exoddus is a bit pricey, too.