I have an installation of WP multisite running on IIS.

I have an installation of WP multisite running on IIS. I've modified the PHP.ini file to allow for 500mb post size and 500mb media file size. When I try to upload anything above 8mb (the original size before I changed the setting) it gives an 'http error' on the crunching phase of the upload. It now displays in the media upload screen that the max upload size is 500mb, but isn't working. Any ideas?