I have five. (PHB, MM, DMG, OA, & D&DG; all 3.0) I had all the 3.0 splatbooks, but I sold them to a friend. (I think those were the first RPG books I ever allowed to leave my possession.)
I also have the 3.0 D&D Adventure Game. I guess that counts as a sixth. Oh, & four 3.0 modules. That brings me to ten. So, I still voted the currect category. Oh, wait...I think I still have that 3.0 Gazatteer around here somewhere.
(If issues of Dragon & Dungeon counted, though... (^_^))
I also have six d20 books: CoC, Star Wars, The Wheel of Time, Soverign Stone Campaign Sourcebook & Codex Mysterium, & T20. Plus a few d20 modules.
I also have the Mongoose Pocket Players Handbook for when I play 3.5. (The only OGL book I have that really seems relevant to this question.)
The 3.0 hardbacks make up 24% of my (A)D&D hardback collection. (^_^)
Edit: Just to be clear, I have zero 3.5 books.