http://mayang.com/textures/
I'm not sure if you guys know about this website, but it's amazing. Basically, if you're not too good at making your own graphic backgrounds with brushes, this is a major alternative. This website has a bunch of free, yet very large, textures, including walls, buildings, metal, etc. By putting & blending a few textures together, you can immediately achieve a background.
Then, throw in a couple of renders, add some effects, and quickly yet effectively, you have yourself a new sig.
This took me no longer than five minutes to complete. Try it out for yourself and I'm sure you'll get a nice result.