Personally, I think the iMac screens look wonderful. They are so much brighter, sharper and clearer than the matte-finish Apple displays they replaced that it isn't even funny. Whatever you do, you should most definitely upgrade your aging G5.
More generally, any high-quality modern computer display will be perfectly fine for doing graphics. If you're designing for the web, what others see when they view your work is going to depend on their screen, not yours. And if you design for print, well... Getting the final print to match what you see on your screen is a subject unto itself.
If I were you, I'd get a new iMac. It's a no-brainer. The screens are gorgeous. Just don't set it up where the ambient light is too bright or you may have trouble with reflections. Actually, too much ambient light is bad for graphics work no matter what monitor you're using.
