Upgrading My SDRAM

I have many questions. I own a Power Mac G4 which I bought in August, 2001. I am currently running OS X 10.4.11. I have 128MB SDRAM in the IMM0/J21 slot and 512MB in the DIMM1/J22 slot and the original 60 GB hard drive. I know I can add another 512MB to the third open slot, but can I also swap out the 128MB for a third 512MB? Also, I want to add another internal hard drive. What would be the maximum capacity recognized by my Mac? I'm attempting to speed the "old girl" up. Can I designate the new hard drive as a slave drive so that nothing else gets written to the original drive? I have issues with space when trying to edit in iMovie. There's not enough room. I've attempted to use my 1TB external drive for editing, but the Mac insists on writing to the 60GB hard drive.