This took me a while to figure out and I thought I'd post for others. The iMac with the radeon x1600 card booting xp in bootcamp freezes HARD on Galciv II, actually rebooting the machine. The problem is that the supplied driver is old, but ATI's updated driver for bootcamp/xp is also too old to work. To get it to work you actually need to grab the regular xp version of the drivers, but they will not install by default. You will need to manually install it ("have disk" and don't let windows search). Then it works fine.