Can I install Parallels alongside Boot Camp? I have a 24" iMac and have used Boot Camp to install Vista on a 100GB partition of the hard drive. Can I install Parallels Desktop as well so it will be easier to switch between Mac and Windows?

I have 4GB of RAM. Is that enough to run Parallels?
Yes, you can install Parallels on your Mac in addition to Boot Camp. Keep in mind that this takes a fair amount of hard drive space, so be sure you've got the space to spare.

As for your memory situation: I run Parallels on a MacBook with 4GB or RAM. It works okay, but it's rather slow. Most things will run acceptably, but performance will be better using Boot Camp.

I should also point out that this setup will be a bit of a hassle. Since the two Windows systems will be independent of each other, any changes you make on one will not copy over to the other system. That means that it could take a fair bit of work to keep both systems synchronized and up to date.