I'm afraid I don't know what 'rules' YouTube impose (if any) on image quality (and therefore file size). It may be that they're reducing the bit rate once you've posted the video (but if your 'preview' is on your local machine, and still blurry, this probably rules out YouTube tinkering).
If you could point me at one of your 'blurry' YouTube videos, I'd probably be able to tell you what the fault is.
640x480 pixels should give a resonable, old skool TV quality picture (basically, it's the quality you'll get on an NTSC TV, vs 768x576 on a PAL TV). So, whilst you'd expect to get some 'softening' of the picture due to the reduction in number of pixels, it shouldn't really be 'blurry'.
My guess is that it's the bit rate setting for the video conversion; look at the settings in your slide-to-video conversion tool.