Super slow PowerBook...

256mb of ram is ridiculously low. That would likely be the problem. You can bump it to the 2gb max for less than $100 I would imagine. My wife's 1.5ghz powerbook was running slow on 512mb. We added a 1gb stick last year for cheep and it really boosted performance. Give it a try.

 

You can run the Apple Hardware Test, I believe this is on the install disks.

If you find something that is broken you would need to fix it yourself, or pay Apple a ludicrous amount of money to do so. Depending on whether this is a 12" Powerbook or a 15/17" will tell us how easy it is to open up.

And yes I would recommend chucking a GB of RAM in there, plus the original 256 (or 128 if its 2 sticks of 128, since you will need to remove a 128 to fit the 1GB.)

 

Yes, it's very low. Follow the advice of the other posters and upgrade the RAM. It'll be like getting a whole new machine.

For my own experience, I still have a 1.25 GHz PowerBook at work running Panther. It originally came with 2x256MB from Apple, and we replaced that with 2x512MB third party memory. We had 2 separate problems crop up simultaneously: the third party memory was crap and had to be exchanged (constant kernel panics), and one of the RAM slots in the machine went bad. That meant that I was down to a solitary 256MB stick of memory for a time. It was slow. Painfully, agonizingly slow. This is because it was constantly swapping to the 4000 rpm hard drive for additional memory.

Given the likelihood that your sister's machine probably has the same slow speed of hard drive, I'm not surprised at all. Give that hard drive a rest and add some memory.

 

It's four years old, maybe the hard drive is going bad...

 

I have the exact same machine, with 1GB installed (only one stick as the other slot is buszteddd) and it's still pretty fast. in fact, for normal stuff, it's plenty fast and it's only really when i am doing some seriously nuts audio processing that it gets a bit mardy and slows down. when i had just 256 (when i first got it) it felt slow and crappy.

 

256MB is not low for Panther, it's too low for Mac Office.

 

Yes, if you want more than the 1.25GB that you'd get with the 256MB plus 1 GB you'd have to pull the 256MB out and it wouldn't be used any more. That really depends on the Powerbook though, because the 12" had 1 ram slot soldered to the board, so you could only add one stick and max out at 1.25 GB. The 15" and 17" don't have that limitation.

Go here:
http://eshop.macsales.com/shop/memory/iBooks-PowerBooks/G4-Aluminum/

Look under the heading, "OWC PC2700 DDR 333MHz SO-DIMM for all other 12/15/17" Models"

Personally, I would definitely upgrade to Tiger, but I haven't experienced Leopard on a G4, so I can't speak to the performance. Search for threads about Leopard and Powerbooks here. I'm sure there's several that discuss performance.