Hi Cornelia,
Glad you caught on to the fact that 'memory' and 'drive' are two different things. In very basic terms, memory (RAM), is temporary and 'helps' your processor (CPU) to operate more efficiently. You have two processors and they should be plenty 'fast' for your needs. CPUs are measured by speed; in your case both operate at 867mhz (new G5s operate at, for example, 1.5Ghz; which is faster). Drives (hard disk drive, hard drive) actually store our stuff. Hard drives have storage 'space'. For instance, one of yours has 57GB of storage space and is only using 23GB, less than half. Then you have optical drives: DVD, CD; you want a DVD/R/RW. There are several Mac-compatible models. But already having an external optical drive, you don't need another. But it can be done; again, fairly easily.
Grant's last post is cool; i.e. adding a HD and memory are easy...after you've done it once. Check out this video:
http://eshop.macsales.com/tech_center/index.cfm?page=Video/harddrive/g4mddata133 /g4mddata133h.html
Bop around at that web site>tech support>installation videos>choose ur machine>memory; for how to add memory.
And, as stated, our Macs are made for us to add drives, memory, graphics cards, PCI cards, and optical drives. Here's another link:
http://support.apple.com/kb/HT1815
So, Grant and I (and others) recommend that you up your memory and replace your 57GB hard drive. And, as stated, if a tech had to modify your machine to add the 250GB drive (shows as 237GB), something smells. And, correct me if I'm wrong, adding a SATA drive would require more than just a new connector. This might be another issue. There's dozens of places to buy RAM; but it has to be correct. You'll see many here recommend buying memory from macsales.com and crucial.com; but only because most that ask aren't knowledgeable enough to shop around for the correct sticks. OWC and Crucial do it for you. For now, one 512MB stick will do you some good; only about $17 at OWC.
Many of us run older Macs; we're almost averse to advising anyone to buy a new machine. As in your case, what you're trying to do with movies, your Mac is adequate and you don't need a new Mac nor a new CPU; unless you really want to go to the latest iteration of iMovie.
When you're ready, post back; preferably at the G4-MDD area. Good luck and please don't be afraid to open you lil ol Mac and at least take a look. Just don't touch anything...lol.