Sorry if these are dumb questions but I've been through the ffmpegX docs AND the mencoder docs AND half of google and not found any beginner's guides on how to do what I'm trying.

What I want to do is convert some VCD-quality MPEG files to H.264 MP4 files. I tried for ages fiddling with the qmin and qmax settings and it always came out appaling. Then I read on an XviD forum that to get VBR you have to use qmin=qmax. I tried that with H.264 and it seems to work, but I'm struggling to control it. Here are my questions...
  1. Am I right thinking that you must always have qmin=qmax to get VBR or do they mean something else?
  2. I've tried various qmin values and 36 doesn't lose much visually on the short test file I've been using (but it isn't top quality). Is there a guide to estimate qmin based on input source? (I should probably read up on PSNR).
  3. While I was testing, I noticed that as I increased qmin, at a certain value the output filesize reached a lower limit, which changed when I tweaked the bit rate. What is the meaning of bitrate in a VBR file?
  4. The XviD article I read said that with VBR, two-pass encoding becomes meaningless because the codec always allocates the correct bandwidth for each part of the video. But when I tried a two-pass VBR H.264 encode, I got a different file size. What does two-pass encoding do in this case?
Feel free to point me to the relevant bit of the documentation that explains all this if I did miss it!!!