Considering using BootCamp is the equivalent of running Windows in native mode on native hardware, you would see zero difference between it and a "real" PC. With BootCamp, you really are booting into Windows - there is no emulation layer. There are even reports of people installing Vista without using BootCamp.
BootCamp is just a boot loader that installs a firmware update to the EFI that allows non-EFI aware OSes to boot on EFI. Again, when you use BootCamp, you really are booting into Windows. There's no code emulation.
Parallels is a bit different - it "virtualizes" the system calls to Windows through Mac. The effect you see is that things may run a hair slower. On my C2D MacBook Pro, I run Windows XP in Parallels all day as a window. I have 3 GB of RAM in my Mac, and give 1 GB to Parallels. I don't notice a performance hit on the Windows side with anything I do (mostly browser and office apps).
Where you will notice a performance hit with Parallels is gaming. Running accelerated games won't work. Since BootCamp boots Windows natively, there are no issues running games in BootCamp.
And now Parallels supports two features to make your life ridiculously easy:
1) Parallels can use the same partition that BootCamp does, so you only have to maintain one instance of Windows.
2) Parallels has a "coherence" mode that allows you to run Windows apps literally side-by-side with Mac apps on the Mac desktop. It basically hides the Windows desktop and start bar, and puts running Windows apps on the dock. This is a really neat feature that words don't do justice to.
As a first-time Mac owner (I've had mine for about 3 months now) I can say that I'm happy. At this point, if you like Mac OS X, there's very little reason not to switch.