My other computer is an iBook G4. The battery is now at 57%, should I invest in a new battery? How much are they anyway?

When you say your battery is at 75% do you mean it won't charge to 100%? Does the charger go green when it reaches 75%?

What you have to think of is how long it takes to discharge. If your getting good use from that battery then why replace it. If it discharges quickly then it maybe best to simply replace.

You could try resetting the PMU to see if that helps, but I doubt it will.

Link to resetting the PMU http://support.apple.com/kb/HT1431

Else it may need calibrating

http://support.apple.com/kb/HT1490

A new battery can be expensive it depends which third party supply you buy from.

Stay way from the cheap Chinese suppliers, thoses batteries don't always last that long.

Hope this helps.

You don't specify whether it is a 12-inch or 14-inch iBook.

You can shop for a new battery here:

http://eshop.macsales.com/shop/Apple/Laptop/Batteries

It's where I would go if I were going to buy one.