I can relate pretty well to your situation. I have a Pentium 3 500MHz, 256MB PC100, 128MB DDR ATI Radeon 9000 AGP, etc. Anyway, I ran XP Pro for a while when it first came out. Then I bought my Radeon(after around a year). I went from an ATI xPert98 8MB PCI, and that helped ALOT. Then, just recently, I built a killer system for a friend, and of course I used it (burnin, installing drivers/programs, etc), and when I gave it back to him and started using my computer again, I couldn't take the slowness. So, now I run Windows 2000 Pro, and it is WAY faster than Windows XP could ever be. It works fine with any game I throw at it (unreal tourn. 2003, GTA vice city, NFS Hot Pursuit 2, etc). So, I recommend Windows 2000, except if you are going to do the things rmrf said, or if your sister isn't too computer literate (because in WindowsXP, most things are wizards). Just my $0.03.