Since I'm a proud G4 owner (see siggy) this is what I have to say:
When you get it, it'll be way fast, way way way fast. Depending on the kinds of apps you use you may want to upgrade your RAM. I do a lot with graphics so I upgraded from 512MB to 1GB and it purrs. 512 will do though as long as you don't use too many large applications at the same time: e.g. Photoshop, Firefox, etc. If you're looking for good inexpensive RAM I got my 512MB stick for $37.99. Hard disk space is an issue for me, of course, I'm down to 7GB out of 37.14GB (40GB).
Now I use Mail, Camino OR Safari OR Firefox for browsers, iTunes, Adium and that's about it and I'm using about 340MB of RAM with those. Everything is fast. To make a standard DVD into an image it takes about 3 hours. iTunes ripping songs is quick, probably like 25 seconds per song depending on how clean the CD is and that. I've ran DOOM 3 on this kitten and it ran at about 30FPS. Programming its good for, it packs a punch with programming applications. You have CoreAudio and CoreImage so rendering is fast for apps that use it (e.g. Dashboard). It's light, I mean like way light, I sit it on one knee and type and kick back into a chair. Lots of apps come with it and run on it. You get iLife '06, Quicken, etc. etc.
Now for my 2 Cents:
Wait till the 9th and see what comes out, sometimes apple will throw some iBooks into their saver section so you can get them for like $800 or less sometimes. Maybe you want an Intel Solo chip instead of the G4. Just depends on your needs and what you will do with it.
--Added note, the Solos should allow for faster RAM and better video card, so if you do a lot with graphics or games or that, it may be better to wait for the solo MacBooks.