I had the same question a few weeks ago. I wanted to buy a new laptop for my daughter and wife to use at home. They will mainly use it for Web, AIM, Email and games. SimCity 4 will not run on our current iBook. It needed a better video card to run SimCity.
Upgrades
The only reasonable things to up grade on current Mac laptops are memory and hard drives. Memory is easy but most hard drives are not easy at all. I would suggest getting as big of a hard drive as you think you may need in 3-4 years.
Memory
You can always add memory later if money is tight. Laptops have fewer slots for memory both the iBook and the 12" PowerBook have 128 Megs of ram soldered on the motherboard and only 1 slot for expansion filled with a 128 meg or larger memory module. The iBook can be expanded to 640 meg the PowerBook 1.1 Gig. I usually buy Mac Laptops with the minimal memory from Apple they charge a high price for their ram. You can go almost anywhere and get a better price. When I shop for RAM prices you get a reasonable deal up to a point then there is a big jump and you are paying a premium for the extra large memory module. I usually go just below the big price jump. On Apple's web site you can an PowerBook with 256 Meg of RAM for $1599. If you up the ram to 1.2 Gig of RAM from Apple it costs $2299. If you buy the 1 Gig memory module from RamJet for $599 you have essentially the same machine for $2198 saving $100 not counting shipping cost for the RAM. Smaller memory modules save less money. Below is the URL for Ramjet:
http://www.ramjet.com/pbg4.aspPros for the PowerBook over the ibook
Powerbook is faster for several reasons depending on what you are doing.
PowerBook has a larger level 2 cache.
Faster Bus for transfering data
Faster processors are available
Can add more Ram than the iBook
Better video card than the iBook
Smaller
Cons for the PowerBook
Gets kind of hot on your lap
More expensive
Might be less rugged than the iBook
Airport reception is worse on the PowerBook
Pros for the iBook
Cost
Larger easier to hold on you lap
Better Airport reception
More rugged that PowerBook
Cons for iBook
Slower Bus
Smaller Cache
Less possibilities for memory expansion
Larger
I ended up buying the cheaper 12" PowerBook and adding more memory. I would recommend 256 is the bare minimum you want to have. I figured that the PowerBook would be a viable computer for a longer period of time.
Hope it helps!
Guy