I also find it runs smoothly and with almost no problems on a Mac Pro/Bootcamp/WinXP with an NVidia Gforce 8800GT. Yes, it's kind of spendy, but on the other hand, it may be more than I need for this app. At the time, I needed the extra graphics horsepower to run X-Plane v.9, and perhaps Orbiter would run OK on a comparably configured iMac with it's built-in graphics.
I enjoy Orbiter a great deal, if I assume Dr. Schweinger has to choose how to spend limited time and resources, I am content to see the vigorous growth, quality, and reliability in the Windows version that we have.
By the way, the same system also does a great job on Silent Hunter IV. I can even run it with acceptable performance under OS X with Parallels virtualization of Windows XP. Have to go to Boot Camp though, when the graphics get heavy, such as entering port or making an attack.