Jumping in a little late on this...
Did you consider streaming the preview content instead of progressive downloads? Especially for the previews - less buffering time and ffwd/rewind capabilities off the bat - not to mention saving bandwidth.
Here is it streamed from a CDN:
http://wmod.cdncon.com/a277/o2/drmnetworkswm/preview[1].wmv
Also, some of the streaming networks have built-in transcoding engines - for one-click transocding into multiple formats, including Flash.