[G4] Need more hard drive space, plus other dumb questions

[Amanda Jacobson Snyder]

Okay, as you'll quickly realize, I'm a newbie here. 

I have a Powermac G4 I bought in 2001 and I THINK it's the Sawtooth version. I recently bought some more memory for it and now I want to give it more hard drive space ... it tells me I only have 1 GB free. However, I'm in over my head now. I don't know where to search or what to even search for! All I can find to purchase is "CPU" and I have no idea if that's the right gizmo. I'm pretty ignorant when it comes to hardware, as you can see. 

Can anyone help? Suggestions on where to buy would be great too. I think my hard drive has 20 GB (I'm away from home at the moment) and I'd like to get at least 20+ more ... have been doing some work with digital camera raw files, and that eats up space in a hurry. 

Here's my other question -- I put the new memory in the memory slots just fine. So are those other slots for hard drive space? 

Thanks so much, 
amanda