the hard drive question

in general, I can use the hard drive taken out from a pc and put it in a mac ?
how about the other way around?

yes I do that once in a while my self, the mac os will not boot on the pc since it is open source that requires the mac decode chip to work, so you will have to reformat it on the pc and depending on the hd you may need a hd reader card, all it is is a storage device with different drive specs and the pc just needs to be able to read the specs, like when people install flash drives in their ipods that is designed for laptop drives

So basically I cannot just go to out and buy a 300GB seagate hard drive right now and put it in a power mac G4 or G5 and expect it work..(?)

Of course you can!

But your second question somehow doesn't seem to be related to your opening post (if you pull a hard drive from a WinBox and try to put it in Mac, it isn't a brand new Seagate...) and microtalk's reply is pretty nebulous to me.

Could you clear this up for us so we can see what we can help you with?