First I would think that if the host is rerendering your files to flash that you should send them to them in flash and maybe they would not rerender them. Then you would control the quality.
Second depending on how long each tour is, a 640×480 would be a large file and would take longer to download and depending on how it was being played some computers would have a hard time keeping up.
Do you have a sample that you can post for us to look at?