Has my hard drive failed, and if so can I replace it myself?

Question

Can someone help me, please? I believe my hard drive has failed (or is about to) and needs replacing. Problem is I took my laptop (Powerbook G4, 15" 1.5GHz 512MB 80G Sdrv APX, recently upgraded to OS10.5) to our local official Apple Store to be serviced when this problem first became apparent. They returned the laptop two weeks later saying it had now been fixed using Disk Warrior, and was working fine – no charge as I bought the laptop there originally.
Great! Took it home, booted up OK first time, worked as it should for a few hours, then shut down unexpectedly. I have tried to revive it several times now, and all I get when I power up is the chimes, then a grey screen for several minutes followed by the dreaded flashing folder with a question mark, which I understand to mean it can't find a bootable system folder.
The background: Problems began when computer shut itself down during sleep (energy saver set to sleep after a time as is normal). I have a LaCie Little Book 320G connected by firewire, using Time Machine to make regular backups. The problem seems to relate to the laptop going into sleep mode after the alloted time and in some way this seems to make the Little Book think it has been ejected incorrectly, and puts up a warning message. Sometimes it is unavoidable to have to leave the laptop, with the Little Book connected, powered up and downloading (eg. podcasts) overnight. When I get back to the laptop in the morning it appears to be in sleep mode (which I would expect) but has in fact shutdown and is frozen. It has to be restarted with the power button, and even this doesn't work first time every time.
This happened regularly over a couple of weeks and I recall hearing a whir from the hard drive, which I'd never noticed before. Eventually the whir developed a few clunks and stopped and started intermittently. At the same time I got the chimes, followed by a grey screen for several minutes followed by the dreaded flashing folder with a question mark.
So what I need to know is does this sound like I need a new hard drive (what else can it be?), and can I fit this easily myself? I saw an item on TV recently which showed how 'easy' it is to replace a hard drive as they are all a standard size for laptops – just open the case, pull the old one out and slide in a new one. Knowing Apple there is probably more to it than this, so what problems should I be aware of, and can I upgrade the hard drive to a bigger capacity at the same time? Mine's got an 80GB drive at the moment.
Sorry this is so long, but there seems to be so much background I needed to give you. Thanks if anybody is willing to help me.
Bruce

Answer 1

Thanks to 'um' and BGreg for the helpful replies. I think I'll go for a new hard drive now. Although I'm not entirely confident inside the guts of a laptop (having looked at the iFixit guide, which does show things fairly clearly) I am less confident with my local Apple Centre for returning the laptop as 'fixed' when it clearly wasn't. They want about £50 to do the work on top of the drive cost too.
I'll get myself the torx screwdriver – not familiar with the small allen 'hex' key. I have a set of allen keys, so I assume hex keys are similar in having the right angle bend but with the star shaped end? Wouldn't the torx screwdriver do the job just the same? Don't see why I would need to hunt down a small allen hex key, but I would hate to end up stripping a screw head by having the wrong tool.
Western Digital do a 250G parallel ATA drive I hear, but only give 3 years warranty. Suppose that's a decision I'll have to make for myself. I've done the "powerbook hard drive replacement" Google search and there's loads to read. A quick glance over one article spoke of the 'horror' of lost screws so I'll find some empty yogurt pots as you suggest BGreg.
Lots to read, then order the drive. After that I've got to fit the thing, and my work shifts don't leave me much free time right now, but I'll drop back online when it's done and let you know how it went – or maybe ask for help if it goes pear-shaped!
Thanks a lot guys!
Bruce