Didn't I read somewhere that iMovie till version 6 was no real HD or was that just AVCHD...?
Anyway, if you still have iMovie4 and younger projects laying around, use iMovie 5, because iMovie6 can't handle the older projects properly (some seconds or minutes at the beginning and the end will be missing).
With iMovie 6 I guess, you will have the newer codecs. For x.264 there are two free codecs from
http://www.macupdate.com/app/mac/20273/x264-quicktime-codec
http://www003.upp.so-net.ne.jp/mycometg3/
The newest codecs would come with Handbrake (only encoding, no cutting). (The version for 10.5.8 can even encode DV files).
Another option to cut+encode DV is mpegstreamclip (no effects etc., but you can use them on iMovie 3 - and maybe 4 - and then use the new file in the project folder, that will have the effect and open that in mpegstreamclip. Adding an effect in iM5 or 6 will not add the effect to the DV file itself or as a new clip in the folder, it will be saved in the cache folder in the timeline-file and can not be used by other programmes).
Exporting a DV file captured in iMovie out of iMovie by iMovie's export/send option will result in a DVCpro file (which will loose quality of the original DV file, hence I use mpegstreamclip to cut and "glue" scenes together, that I have captured with iMovie. I take them out of the "media" folder inside the project folder, that can be accessed by mouse click + ctrl -> show package content).