All I can tell you is that I’m running CS3, CS4, CS5 and CS5.5 on Windows 7 using Boot Camp on a MacBook. There’s no benefit for me to using Parallels so I have no experience to share with you in that regard.
Depends how powerful your machine is. Like all emulation, this will require quite a bit of RAM. Beyond that you probably won't be able to use graphics hardware acceleration via OpenGL. Well, you can always try. Nothing's lost and you can always still go the Bootcamp route...