PC2700 is generally backwards compatible with PC2100 RAM, but Mac laptops are known to be quite picky. It would be best if you could "test out" the RAM so that you could opt not to get it (or get the more expensive PC2100) if it doesn't work too well in your iBook.
Don't scrimp on RAM, though, as bad memory will definitely ruin your day. It pays to get quality RAM, like those from Crucial/Micron, Kingston, and the like. There are lots of satisfied customers from vendors such as Trans International, MemoryX, and the like. Visit ramseeker.com for more info.