The 500d's only improvements over the 450d are a higher res LCD and a digic4 processor. It has more megapixels, but in my opinion, this isn't really necessary and will create more noise etc.
As a 450d owner, I would highly recommend it for functionality and image quality. I do wish I had a 40d though, because the feel in hand of the 450d isn't too comfortable (although I have a battery grip which helps a lot).
There's some software you can use to get video from your 450d. It doesn't have sound, and isn't 1080p, but if you want to experiment with video on you camera you could try it. I've got it and can be fun to play with the DOF with my lenses in video.