Rmagick image resizing problem

I am using attachement_fu plugin with Rmagick image processor for
uploading images. Even though I am able to upload images properly there
is some problem with thubnails created. Thumbs are created with same
size as original image. Also the height and width of images are not as
null in database. Here is my model.
  has_attachment :storage => :file_system,
                  :resize_to => '640x480',
                  :thumbnails => { :thumb => '160x120', :tiny => '50>'
                  :max_size => 5.megabytes,
                  :content_type => :image,
                  :processor => 'Rmagick'