Hi Peter
Karen is correct. Make sure your new PCs have dual DVI connectors, some will come with two as standard, depending on the graphics card installed. And also check your monitors have a DVI input if you're buying them separately, some of the cheaper ones still only have VGA (but you can get adapters if you want to use existing monitors.)
I've got a twin 19" arrangement on my desktop which gives great flexibility. I can put whole applications in the second window, or just palletes. I generally have my email in one screen and I switch between my other apps on the other. We don't print any emails, so it's useful to read from Outlook while working on something else without switching applications.
Make sure you get matching monitors too!