In order to make the panel bar more transparent (as transparent as you like) follow these instructions:
Go to 
system -> 
preferences -> 
compizconfig settings manager. (or, if you are using the Win7 look, go to 
start -> 
control center ->
compizconfig settings manager).
Scroll down to the 
accesibility field and click/open the 
opacity/brigthness and saturation button.
On the first tab (opacity) at the bottom of the page click "new".
In the box that says "windows" write 
- Code: 
- name=gnome-panel
Set the bar below (window values) to any number you prefer. 100 is for no transparency at all - 0 is for full transparency.
Personally I have set it to 90. I think it is the best setting as you get the transparency of the bar and it still remains easy to read! 
