Rahul Sood has posted some benchmarks in his blog of an Intel powered Apple iMac running Windows XP. There were some graphical anomalies, and overall Rahul characterized performance as being similar to a "mid-range notebook". Not earth shattering performance by any means, but interesting info nonetheless.
"We finally benchmarked the Apple PC in Windows successfully, thanks to the latest video driver release. It seems to struggle a bit in games with the level of detail increased, but it's certainly not terrible, it's about as good as a mid range notebook system. We also noticed artifacts in certain games at higher resolutions. It could be related to heat or a driver issue, at this point there's no telling for sure."