You'd need a DVI switch as least. A KVM switch (which switches signals for keyboard, video, and mouse) would be better. Then you could have one set of peripherals on your desk for both computers and could just switch between them.
EDIT: It really depends on whether or not you run your PowerBook in lid-closed mode at the moment though. If you use it open w/ the screen, every time you switch to the other display (at least with a cheap KVM switch), the Mac will think it has been unplugged and switch everything over to the PowerBook's screen. Actually, a PowerBook complicates this a bit. I've only ever done it with a Dell display that had a VGA input too. So I put the PowerBook (in lid-closed) on the DVI in, and the PC on the VGA in. When I switched that, the monitor was never disconnected so I never had problems. Not sure if the cheaper KVM switches are the same. You might need to do some research on them.