800mhz ibook would be "suficiant" for most applications.
It's not like it's going to rock the house, or anything like that. If you have a fast desktop you will notice that it is a bit slow when opening apps and such. It depends a lot about how sensitive you are to that sort of thing. Having lots of memory can help out a lot, too.
The 800mhz G4 is about equivelent to a 1-1.2ghz pentium desktop cpu or so. That's a rough estimate, in somethings the the g4 excels and in some things the intel is much better.
The thing that makes up for it is the fact that unlike most laptops in that price range it has a decent ATI video card, instead of the on-board intel stuff. So basicly your trading much faster Pentium 4-m for a better video card. Which, since OS X uses opengl for acceleration, is a acceptable trade off IMO.
What makes the Ibook a good deal is it's size, weight, the quietness of it, the lower operating temps, battery life, and the extra firefire/usb supports. It won't make a good desktop replacement setup, but it's small enough and handy enough to carry around with you. Low-end Dell/HP/Gateway-style laptops give you a fast proccessor, but tend to be bulky, hot, and loud.
However 800mhz is getting quite a bit long in the tooth. The way I figure it the deciding factor is going to be how you intend to use it, if your going to be carrying it around everywere and mobility is what you want, then the small 12 inch size and light weight is great, but if mostly your going to use it on a desk at home or at work, then you may want to find something else with a bigger screen and faster proccessor.
(just no matter what you decide... aviod anything with Celerons cpu. Even 2ghz+ ones don't compare favorably to the G4 when you take the heat and power draw into concideration.)
