Could be that your server is running suphp, in which case you can't have php directives in the .htaccess document (doing so causes 500 internal server errors).

php.ini should have something like this in it:

Code:
max_execution_time = 600
max_input_time = 600
memory_limit = 32M
register_globals = On
post_max_size = 800M
file_uploads = On
upload_max_filesize = 800M
remove any "php_value" lines from the .htaccess and put it in the php.ini without the "php_value" and put an "=" (equal sign) instead.

for example, where it would say php_value upload_max_filesize 100M in the .htaccess, you would remove that and then you would put upload_max_filesize=100M into php.ini

You may also refer to this thread viewtopic.php?f=3&t=295