which iMovie can i use on a g5 dual processor

iMovie '09 (version 8) provided your processors are fast enough. See: http://support.apple.com/kb/HT3876

for details.

Geoff.

According to this:

http://www.macrumors.com/2013/10/22/some-users-with-older-copies-of-ilife-and-iw ork-having-difficulties-upgrading-to-new-versions/

it is available from the App Store for $15. It doesn't show when I launch AppStore but that may be because I already have later versions. You will have to search for it on the web. As part of iLife '09 its pretty old now but Ebay appears to have some copies available.

Best of luck!

Geoff.

iMovie 9 is a nightmare. Stick with iMovie 6.