The Aluminum PowerBooks require the 65W charger. Titanium Powerbooks, except for the 1GHz DVI use the 45W charger, but will step down a 65W.
Long term use of the 45W charger can damage the charging circuitry of the Aluminum PowerBooks.
You'll notice that you maintain a charge with the 45W charger, but while using the Mac it will not actually charge. It WILL charge if the Mac is off or asleep but it will be very slow.
Do yourself a favor and use the 65W. That's what the spec is for the Aluminum PowerBooks.