I switched from a G3 to a G5 about two years ago, and I saw an amazing difference, and the new Intel Macintosh systems are screaming fast (even when compared to my G5.) I'm not sure what OS you used on your G3, but I was using Panther (OS X 10.3) and I love the new features of Tiger (OS X 10.4) that came with it. Plus, Leopard (OS X 10.5) is coming out this spring, so it will be a great time to upgrade. And, you'll love the other new features like Front Row and Photo Booth (entertainment that will last you a long time.)
Really, these intels are so fast. I think you'll really love the new features of Mac OS X. My iMac G5 was one of the best purchases I've ever made!
(Oh, and if you're in college or if one of your family members are, you can get an education discount which saves you at least $100 on the 20" iMac!)