Since you're getting the Aiport Card and larger hard drive, why not consider buying a stock PowerBook? The difference in cost is only $346 (could be even less if you use EDU or ADC discount) and it comes with a built-in Aiport Card and 60GB hard drive. Choosing the PowerBook, you get a faster, more capable machine thats compatible with upcoming Core Image in Tiger (iBook isnt) and its also lighter than an iBook. As for the battery life, I don't think theres a big difference, maybe 30min or less, unless you know otherwise.
To address your question though, yes I think it would be comparable if you're just using it for Word, surfing and light loads, but anything beyond that I would bet the PowerBook would outperform.
Lastly, don't buy the RAM from Apple, you can get it cheaper elsewhere (ie. crucial.com).