that brown one looks pretty kool
but most of the others look kinda ify, i really done like then one witht he flame BG you could have done more with that.
iuse stocks or 3D abstract renders for your BG.
you can get abstracts BG's from http://planetrenders.net/renders/
with those youl be able to do so much more
also try adding border it can make sigs look so much better