Resolution of Frame Grabs from Sony HDR-HC7

I've been using TRV900's as a tool to shoot video along reef transects since 1998 as a component of various reef monitoring projects. Very satisfied with the cameras and housing (Amphibico Navigator) but we're looking to upgrade to HDV to improve resolution of imagery, to allow more detailed analyses of species composition in video frames. Digital stills not an option due to in-water bottom time constraints - you can cover alot more bottom with video than a still camera in a dive). We currently take the raw video data shot with progressive scan and do individual frame grabs using iMovie on an Apple G4 with a dual processor, ending up with 640 by 480 jpegs (may end up with hundreds of frames per transect). Then we review individual frames with image analysis software. What would be the positives (and negatives) of going to an HDR-HC7? Or any other HDV model?