Can anyone recommend a slightly better rear derailler that will fit my bike?

I need a new one. The one on my bike, a Trek 820, was a Shimano Acera which I've only seen for sale on the web at AE bike for $20. Only problem is AE Bike won't sell Shimano over the internet.
I was thinking of upgrading to a better one if that is possible.
Also, this is a 7 speed bike. All the deraillers I've seen for sale are 8/9 or 10 speed deraillers- this includes the Acera that is being advertised at AE Bike.

Can anyone recommend a slightly better rear derailler that will fit my bike?