Best Configuration for attachment fu (images)

Hello,

Well I'll like to know which is the best configuration or the most used (standard) when uploading images via attachment fu (or related). In particular I'll like to know which is the best resizing option and the best "thumbnail" size for large images. This is my current configuration. When I resize strictly to 640x480 the image loses its original ratio.

  has_attachment :content_type => :image,     :storage => :file_system,     :max_size => 4096.kilobytes,     :resize_to => '640x480',     :thumbnails => { :thumb => '32x32', :medium => '96x96', :large => '320x240'},     :processor => :ImageScience     .

Thanks,

Elías