Question about Macbook Hardrive

I have a late 2005 15" Aluminum Powerbook G4, 1.67ghz, 2gb ram and the 7200 rpm hard drive. The computer is blazing fast and still running 10.4 Tiger. I never bothered upgrading to leopard since that's as far as it will go anyway since snow leopard isn't supported on PowerPC's. I have been looking at some used MacBooks on eBay. Mainly around 2008-09 models with 2.4ghz Intel core 2 duo, and I believe they all now have 5400rpm drives. Will I notice much of a difference in "downgrading" to a 5400 drive in a used macbook with 4gb of ram? Why doesn't Apple still offer the 7200 rpm drive like they did back in the Powerbook days?

You won't notice much if any difference since a Core 2 Duo MacBook with 4GB of memory is going to be faster than your Powerbook.

I believe the reason Apple didn't offer the 7200 RPM drives until lately is because of the louder noise, heat, and additional battery energy required. I don't know why it was an option for your Powerbook.

I put a WD 7200 RPM drive in my 2008 MB Core 2 Duo and as far as I can tell it made no difference whatsoever for heat, noise, and battery life.