Thanks for the answers! It looks like XP Home Edition is good enough for me, so like you said, I'll only get Professional if the price isn't much different. I'll probably do that when I get a new computer. My current computer is 800 MHz AMD and I think I'll get something faster within the next several months, in time for when SEV comes out.
I'm running Windows ME right now and it doesn't crash too often, maybe couple of times a week as long as I'm careful. One of the annoying things however is that I cannot switch between SEIV and any other program during the AI's turn. If I use ALT-TAB it crashes and have to reboot. Also for some games if I quit, start a game, quit and so on several times it gets slower and slower and eventually it just unexpectedly quits the program and doesn't run anymore. I periodically have to shut down and restart.
I'm considering getting a notebook computer, maybe an IBM Thinkpad for my next machine. It will be useful to get something that's portable.