Can I upgrade my fan to a newer style, unibody fan?

The old school MBPs usually run very quiet. Mine (2.4GHz Santa Rosa) has the fans running idle (appr. 2000 rpm) and is barly audible even under heavy load. When these machines get noisy it's A: dust in the cooling system, take the fans out and blow out any dust that has accumulated behind them and/or B: Worn out fans. Up to a certain state you can still recover them by relubricating. Not so good video of the process here. Place a drop of silicone lubricant or similar into the brass bearing. If that fails you'll have to change the fan(s).

When you're at it it's a good idea to change the thermal compound. You'll have to remove the logicboard in order to get to the heatsink, but it's worth the trouble. In my opinion Apple uses inferiour compound in too generous quantities. I use Arctic Silver sparingly, and it always works wonders.