I've tried to load as many texture into the 64bit version of Celestia as I possibly can. I could handle well over 5 GB of RAM usage, before I had to kill it because it started to use my hard drive instead. So in principle I am able to load pretty much the whole solar system now without having to worry it crashes
But even so, Celestia becomes very laggy if you hit the limit of your RAM, so it would definately benefit from deleting unused textures from the memory.
I don't know the specifics of Celestia's usage of tetures, but I think it loads it to the graphic somehow. But maybe not always, since I can load more data than my graphics memory as well as non-power-of-2-textues. But it used to crash when trying to load textures larger than my graphics supports (16k). I haven't tried it with this version, because obviously I don't have any single texture that large in my installation left.