Adventures and "setting" books are far and away the most useful products to get, for any edition. So I'll keep buying stuff from Paizo, Necromancer, Goodman, Green Ronin, etc..., no matter what edition or system they are published in.
System neutral would definitely have the strongest appeal to me, but I won't be excluding anything else.
Since I DM a lot I also find Monster Books very useful, but I doubt I'll be buying 4E versions.
Other than that, I have a ton of other books, and will be keeping them as sources of inspiration. Thats what I have liked about many books in 3E. It detailed aspects of the game that have never been really addressed before. So I love stuff like Book of the Righteous, the Legends and LAir series, Monte's books of ...., Goodmans Compleat books, etc...
Not to mention the stuff by Fiery Dragon, Fat Dragon, WOTC Map Tiles, etc... Never have I had such cool counters, map tiles, and other "accessory" type items to enhance my game with. Heck, even music made for games!
Plus some seriously awesome boxed sets! Rappan Athuk relaoded, DCC 35, DCC 51 Castle Whiterock, the one for Midnight, Wilderlands, Hall of Mirrors, etc... Especially the super well done beyond my imagining City of Brass boxed set. Definitely the King of the boxed sets for 3E.
Lots of good memories from lots of great products from 3E.
Plus Paizo is off to a great start off with 4E with their PF setting materials and their modules, not to mention what they are publishing for Necromancer games, Tegal MAnor, Eamonvale Incursion, and so on. Plus I am sure we can all count on Goodman Games to keep on doing their stuff.
I may not being adapting 4E as my system of choice, but I look forward to seeing what goodies are made for it.