Ubuntu is probably the best place to start.
Very good package manager, good hardware detection, great community.
Ubuntu Site
Support Forums
Ubuntu, as with some other distro's offer a Live CD, or Desktop CD as it's now known. This allows you to try the OS before you install it. Ubuntu's Desktop CD is both a Live CD and Installer. Some other distros have separate CD / DVD's for Live and Install.
If you want Beryl out of the box then look at
http://www.sabayonlinux.org
How difficult it is to get Beryl running depends on your hardware. If you have an Nvidia card then it's a piece of cake.
Dual booting is not an issue. The Installer should ask you whether you want to, however I recommend you read any and all documentation / read the support forums before you hit the install button.