Hey,
I agree that the update from iMovie 3 to iMovie 4 wasn't very ground-breaking, and initially I felt the same way about iMovie 5. HD and widescreen meant nothing to me. However, I did purchase it and I decided to just give it a try playing around with a few minutes of raw footage I had sitting on my hard drive. This was on a 1Ghz Powerbook, with only the 256MB stock ram. My 512MB chip had recently gone bad, so I was expecting iMovie to hang and beach-ball-bitch with only 256. Shockingly, it handled transitions, cuts, sound pastes, rendering extremely well. No crashes or major hangings. I was quite impressed with the background performance changes with the update to '05, and can't wait to see how it will be when I get a new 512mb ram chip back in there.
If you do much editing at all, I highly recommend the update for iMovie & iDVD, especially since you'll be coming all the way from the '03 versions.
Hope this helps,
Josh