I just installed Call of Duty Black-Ops on my Bootcamp (Windows 7 Ultimate, 64 Bot) installation which works fine under Bootcamp, but not in Parallels 6. Parallels was advertised with 3D game support. Why doesn't Black Ops work? And which games DO actually work?
Counter Strike works, so does a lot of Steam games including Starcraft 2 and Supreme Commander. But Total War Games were all basically unplayable when it was trying to render the battlefields. I think they're working on it. But if you can, play games through Bootcamp - as modern games are beasts on your machine (check sys. activity monitor when your're playing) let alone when you're running two OSs at the same time, its probably not a bad idea to play it solely through bootcamp. Furthermore, the network adapator seemed to stutter periodiaclly (about once every 20 min) on my machine (might have been my net connection or the machine itself struggling) but other OS native games seemed to play fine even with my Windows Virtual machine running. Oddly some had no issues running through Parallels either.
