Can you improve the quality of a avi file?

[Findedeux]

My question isn't quite as basic as it seems. I have been upping the bitrate when I make movies (to 2450, and the audio to 224-338) and I was wondering whether files that are not firsthand can be improved by converting the avi to a higher quality MPEG. I only ask because I have been doing tests with some movies and the difference seems to be nil. For example, bitrate from 1150-->2450, with motion search precision on high quality, and noise reduction on (though I don't think I need it), and still there doesn't seem to be much difference. I have been viewing the difference on my computer rather than my TV (in the shop) but shouldn't I be able to tell anyway? The sources I have been using are all movies from Kazaa. Thanks for any comments.

[Findedeux]

Ok, you can ignore my comment. I checked, and though the preview for VCD's on tmpgenc does not work, I can see a big difference when I decrease the bitrate, so it should work the other way, though it seems to be a lot less dramatic.