In case you were thinking of upgrading to WinXP x64 Edition and want to play SC3, you are out of luck. HardOCP tried to benchmark the game and couldn't install the game. See: http://www.hardocp.com/article.html?art=NzY1LDY=
That's weird. I have a new AMD 64+ laptop and I was tempted to upgrade to XP 64. I don't think my hardware can run Chaos Theory anyway but I wonder if other games can face the same problem...
If you're a gamer, I wouldn't bother upgrading to WinXP x64 Edition as all games will support 32-bit for the forseeable future. The only reason I would upgrade is if a new game comes along that you play every day and there was a significant performance boost playing the 64-bit version.
Check out the rest of the HardOCP article and you can see that there is no advantage in virtually every one of the current batch of games.