Guess I'm the odd man out, I almost always sell or trade the old versions when I get an upgraded one. Sold my PS2 KH games last year when the HD sets were announced, same w/ my PS2 MGS games, ICO, the R&C games, the J&D games (except for Jak X Racing, which of course wasn't ported), etc. It's very rare that I'll keep multiple versions of a game, and if I do it's because I *really* like the game. Heck, I even sold my DC Skies of Arcadia because it was redundant with my GC copy.
I currently have the PS2 Okami as well as the PS3 HD port, and I'll pick up the Wii port once it gets cheap enough. I'd also like to import the physical PS3 copy that's in English. Oh, and I still have my PS2 FFX/X-2 even tho I have the HD set. Those are the only ones off the top of my head that I can think of that I kept.