With modern (2006 or so) hardware, virtualization is easier, safer, more convenient, you name it; I don't know why anyone would duel* boot any more, if they have more than minimal hardware. There are many hypervisors to choose from. I understand Xen is a fine choice, though I have no personal experience with it. I have used VirtualBox and VMware. I prefer the former.
(* Yes that's a joke, for those of us who can spell.)
You do indeed run both OSes simultaneously. You'll boot to the primary (host) OS, and when you want, launch an application to install or run guest OSes. The guest(s) will run in their own windows, much like any application. Guests can be constrained to run in a reduced portion of your hardware. On a four core, 8GB machine guests can be made to run as a two (or one!) core machine with 2GB of RAM, and they will run just so, with nearly the same performance as they would on raw metal with those specs. Their "hard disk," by the way, will be a file, a very large file, in your host OS's file system.
Just a couple of things to look out for. You really do want hardware that supports hardware virtualization (VT-x on Intel, AMD-V on AMD chips). Some hypervisors (like Xen) can run specially modified guest OSes on lesser CPUs, and that's commendable, but you'll be happier with hardware support. You'll need RAM enough to cede to the guest OS. Find the option to allow guest OSes to run in (host) swap space and turn it off. Allocate enough RAM to the guest that it won't need to swap internally: 2GB minimum for Ubuntu with Unity or Windows Vista. Windows XP and just about any other Linux OS/Desktop will run fine in 1GB. (Xubuntu for example works fine in 1GB, if you don't make great demands on it from within.) In any case you can probably adjust the RAM and CPU afterward. Also I suggest allocating all disk space for the guest OS's hard disk image up front when you install the guest, when you're given a choice of doing that or allowing it to grow.
Ah! Harrumph -- I should say that video performance will suffer in the guest. Gaming and 3D won't work well. Even so, on my desktop box with oomph, where I keep Flash confined to a guest OS, streaming video on Flash is entirely watchable full screen.