I only have iMovie06 HD, .. .
when your Mac is that old, it came with 'antique' iMHD6 only, double-check, if it full-fills the min. specs of M5, esp. the graphic-card!
e.g. I invested into 8GB Ram, to make the new stuff (FCPX, M5, Lion) 'useful' on my old hardware (MacMini) ...-