While uploading large files through Sitefinity, particularly large ones, such as videos, some settings in the web.config need to be adjusted.
As I blogged in Video in Sitefinity, maxRequestLength is one setting that plays into this, but there is also executionTimeout.
For files in the 450MB range, I've set the timeout to 1000 seconds.
<system.web>
<!-- Sets the allowed upload file size -->
<httpRuntime maxRequestLength="1000000" executionTimeout=...