Dude, I get more use today out of my 2e Planescape books than I ever did out of, say, the 3e Arms and Equipment Guide.
I'm keepin' 'em. At the very least, they're a trove of things to mine for 4e, ideas to plumb. Somone sees a 3e PrC they want, I turn it into a Paragon Path for them, 3e has enhanced my game. YAAAAAY.
I held unto some campaign setting books and edition neutral stuff (minis, battlemaps). Most of the crunchy books and splats I traded off a while ago though. I have nothing against holding unto books for nostalgia sake, but I live in an apartment and space is a premium.
Between the SRD and playing the game for the last several years, I don't really need the books to run 3.5 anyway.
My answer: I look at the piles of crap I wasted my money on I have and I just want to throw it in the trash and start anew. The money I'll get from selling it probably won't be enough to cover the hassle.
Keep it. Reference it. Convert some bits here and there. Use the pics for visual aids (I still pull out my 1e books for certain monster pics, e.g. galeb duhr and tabaxi).