Upgrading G4 1.67 hard drive

Hi all,
been a while since I was last here, but yet again my macs been totally reliable and simple so haven't really had anything to discuss until now.
I was tempted to buy a new 13" macbook pro, but on a test run I found that it didn't really offer me anything I needed over my powerbook.
So I have decided to treat the powerbook to a tune up.

First was the Battery, only 15mins life in it now so a new, official apple, one is on the way. Had for a bargain from ebay.

Second the ram, always had 1gb, time for 2gb also on the way. Not quite such a bargain as old ram seems to be expensive.

Finally the HD, can't decide which one.
Research suggests that the wd scorpio blue 500gb with the ata interface would be perfect.
What I'm not sure about is if 5400rpm is enough.
I'd like to consider a 7200rpm option as well, but I need to get an idea of performance boost vs battery life reduction.

I don't really need any guidance on how to do it, I'd consider myself competent, just some help choosing the right drive if you please.

Thanks in advance for your help,
Elma

Ah,
forgot to mention.
While it's in bits should I replace the PRAM battery.
Have tested it as per Apple guidelines.
It does retain date and time when laptop is off without the battery or charger.
However it is 5yrs old so can't be in peak condition.
As I'm taking out the HD it seems a good point to replace it even with it functioning correctly.
Does that sound wise?

Also I will probably replace the superdrive whilst the laptop is in pieces.
It's not broken, but it is very noisy nowadays.
With this in mind I don't think that the PRAM will ever be easier to replace as I don't plan on opening it up ever again if I can get away with it.

Got the hd wrong actually, wd 320 gb is the biggest in pata.
What about the pram though, is it worth replacing?