I recently just upgraded a G4 533 (Sawtooth) to a PowerLogix 1.4, even at 533 using Tiger it was "snappy," it's now "snappier" and like another poster said, with the big cache the G4s had, quite adequate machine. I own a G5 2.0 DP and the G4 for many tasks isn't far behind at all, if not almost even (just eyeballing it) in some tasks. For day to day stuff (Mail, Firefox, etc.) the speed differences just wouldn't be enough to matter. Depends on what you're doing with it.
A computer is a tool for a job, the G4 1.25 DP is a mighty fine tool.
That being said, the 1.83 is a dual core chip, it has potential do be a monster but realize, until much software is coded to take advantage of this we won't know how big a monster. The the 667 MHz busses and RAM in the Macbook, any native Intel apps will, in fact, be faster than your current G4. OS X, iLife, and anything else, will run appreciably faster. As someone noted upthread, non-native apps are the big thinker. Adobe Apps gamble will be how well Rosetta really does work, as would others. Hints have it that processor intensive apps suffer the most, so video and audio take the biggest hit in Rosetta. To wit, a developer I'm well acquainted with did a test on his Apple leased Intel machine six months ago, Photoshop bogged terribly on a filter test but Dreamweaver (which is so much more text/line based) had very little hit performance-wise through Rosetta.
I too will be buying a Macbook, this year, but only when Apple stops being cheap and maximizing their profits by putting slower chips in there (the 2.0, and 2.13 Yonah chips are out, Acer is using them, Apple bought the cheaper slower ones so they could screw new adopters for maximum profits). I'm waiting for 2.0 just for geeky aesthetic, bragging rights, and my own stupid "I own a 2 GHz machine now and don't go backwards" even though the "GHz myth" is, in fact, in part, truly that... a myth. Processors, busses, the software and OS that's running on them and hundreds of other factors actually equal speed, NOT the MHz. An analogy: a four cylinder Porsche can dust an eight cylinder (cheesy) Camaro handily despite the (cheesy) Camaro having twice the cylinders and more technical displacement.