Sorry, didn't realize you were such a n00b!

Welcome back!
If you plan on getting a drive larger than 1GB you might need to get a 3.1 ROM chip. You *may* be able to get it to work with your 2.0x ROM, so just grab the smallest/cheapest 2.5" HD and see what you can do. I think I got a 2GB drive working in my A600HD. On my other 600's I got a 4GB and 6GB working.
Yes if you got a 10GB drive, you can just partition 4GB and leave the rest of it unpartitioned and you will be fine. Problem is you *may* need later HD Tools to get a larger drive recognized/working. Get HDInstTools from Aminet, that works great.