Depending on what you might want to do there are different options.
TLDR:iMovie 09 for simplicity. Final Cut Studio for 24fps editing with fine grained control.
iMovie 09 will run just fine on the G5 and has the advantage of having a built in library to organize all of your clips, plus you can do more advanced things like better color correction or green screen. There really isn't a whole lot that Final Cut Express can do that this can't. If simplicity and flexibility are your top concerns, I would use iMovie 09.
Final Cut Express has many advanced features but a much steeper learning curve, but it only supports 30/25 fps like iMovie. The major advantage that FCE has over iMovie 09 is that you can adjust things with much finer detail, but with iMovie 09 you don't have to render like you do with FCE and iMovie HD.
If you want to do more advanced editing and have support for projects that use a 24fps framerate, I would recommend Final Cut Studio 2. It's what I use for my editing and it works just the same as Final Cut Express. It has better color correction tools, better customization for setting up sequences, plus you get advanced tools for typography and graphics with Motion.
[doublepost=1545237609][/doublepost]PS: If you're having trouble finding software, I would recommend purchasing a used copy on ebay. Especially if you think you might want to get into the more advanced software of Final Cut Studio 2 as a full boxed copy will have lots of documentation to go with it.
