Eddie
In short, any old IDE hard drive should do the job. Also known as ATA or PATA. Ultra ATA will also work on your machine.
Note drives larger than 128 GB are not natively supported.
I don't know much about the current market, but I would personally look for another similar Mac as a great source for parts and learning.. You should be able to get one quite cheaply and the drive in most G3 or G4 will be compatible.
If you are adding a 2nd drive, the Mac's existing cabling will usually be suitable for connecting up. Making it physically secure may be more problematic.
Note when using 2 IDE drives on the same bus (connected to the same socket on the motherboard) one must be set to 'Master' and one to 'Slave'. This is achieved by jumpers on the actual drive.
For RAM, look for PC100 or PC133.
http://www.everymac.com/systems/apple/powermac_g4/index-powermac-g4.html is a good site for checking the specs of your Mac.