Fast enough for most tasks... what do you intend to use it for?
I use a 500 mhz G3 iBook, with the slower 66mhz bus and the smaller 8mb video card. It is fast enough to comfortably run Mac OS X, Office, Safari, Mail and a couple of utilities all at once without any real delays. It's good for games, except when it comes to Quake3 level 3d games. It runs PhotoShop and Illustrator okay, though I reboot into Mac OS 9 when I have a lot of design work to do, as it makes Photoshop much smoother.
I also do a Java programming unit, and use DreamWeaver MX.
All of the major apps take about 5 seconds to load ... but this is forgivable and they perform well once they've loaded up.
Is there anything special you will be doing with it that might be pushing the iBook much farther than that?
On the other hand, I just realised you said you need a computer "in three years" ... why don't you put off getting one for a couple of years if you haven't got much cash to spare right now?