Uploading Images

I installed wordpress in a server runing Ubuntu Server 8.10, while the database is runing in a diferent server, no problem there, the site works and I can make posts, but when I try to upload an image in a new post I get this error

Unable to create directory /var/www/wordpress/wp-content/uploads/2009/04. Is its parent directory writable by the server?

I gave the directory 775 but it still fails I also tried to do
chgrp www-data /var/www
chmod 775 /var/www
but no result either….
any ideas?