How do I fix/identify a leaky brass gate valve?

Hi! I have three brass gate valves surrounding a filter for my well. They are starting to show some signs of leaking. I realize that I might need to replace them… or, there seems to be a couple of places where they could be adjusted. Is this something I should do? Or, if I need to replace them, can you help me put a name to this type of valve so I can have an easier time tracking them down?

Here is a photo:

http://www.flickr.com/photos/creativehaze/5564992908/

Thanks!