I have been running Windows Vista Ultimate 64bit for the past few days with few complaints. In fact, my only real issues have been an inability to synchronization with my Palm OS-based Treo 650 smartphone (lack of 64bit driver) and a lack of a 64bit DTS Interactive resampling utility for the built in audio controller on my motherboard. The latter is a relatively minor gripe. Dong has complained about some issues with his computer restarting after it's been sitting for a while when running Vista Business 64bit but I have a feeling that it is more power, power supply, and overclocking related.