You can use newer monitors but there are different ways to connect.
It all depends on the video card installed in your computer. Different model cards had different ports. My G4 MDD has the ATI Radeon 9000 with one ADC port (abandoned technology) and one DVI port (useful technology). So we need to either see the description of your graphics card from System Profiler >Graphics/displays, a photo of the ports on your card, or both.
As long as you find a monitor that shares the kind of ports on your Mac, you will be fine. I have a 2008-vintage LCD monitor on that circa 2003 MDD of mine. The MDD shipped with an Apple DVI-to-VGA adaptor so I can connect any monitor with DVI or VGA ports.
There is an inconsistency in your equipment profile: No PowerMac can run OS 10.6. Please do "About this Mac" and verify your processor type and Mac OS version. Then you can edit your profile to avoid future confusion.