A guy who posts on the gaming forum NeoGAF has a reputation for digging obsessively into games he likes to find all kinds of old concept art, cut content, little snippets of story, and so on. He just did it for FFIX, and he found some pretty awesome stuff.
More specifically, he found a ton of art assets that made it into the game as pre-rendered backgrounds. Only these renders are high resolution, at least compared to the PS1's 320x240 resolution. Much, much higher.
Do the actual renders still exist on some Square Enix file servers somewhere? Maybe not, or maybe they didn't use such hi-res assets for the entire game. If they did, we probably would've seen a FFIX HD re-release by now. It all still looks very much like 2000-era CGI, but I think the style holds up remarkably well at a higher resolution.
Check out the whole thread here. There are probably close to 100 images, including a lot of concept art that may be pretty rare or only available in a Final Fantasy art book.