Just had a quick look with Firebug... unfortunately it is not so simple, because of the div dependencies and the background image technique (i.e css sprite). So, just changing the header div height is not the complete solution here.
Changing the background sprite graphic is also not ideal, because you will have to change all other appearances. The other graphics would be shifted.
Background:
a) I think its better to create a new and seperate graphic file for the header. And replace the sprite technique.
b) Or use CSS3 properties. Background color and linear gradient. Rounded borders.
Both ways require to adjust the logo DIV properties as well. change the logo height to fit into your new header height. Because it pushes the menu to the right!
The DIV #menusearch should get a proper margin-top. Push it back until it lands again on the green border.