I've been using Celestia for a few months, and all I can say is that it's one awesome program. It barely chugs along on this old computer, but I'm building a new one and I can't wait to see how she flies on it!
Now:
AMD K6-2 @ 500 MHz
128 MB SDRAM
Voodoo 2
Soon:
AMD Athlon XP 2500+
512 MB DDR RAM
Radeon 9600 Pro
Maybe now I can turn on galaxy rendering without "frames per second" becoming "seconds per frame".