I think learning the box model and observing it's rules will be a big help for you.
See the box model here. The box model says that whatever you put inside an element cannot be larger than that element. margin/padding/border all count when figuring width/height.
Just for example, You have 340px wide #headerleft containing 340px wide .logo which contains 382px wide gl_logo.png.
I think you have a little divitis there too.
Your request #1 is another example of not being familiar with the box model. On FF8 and IE9 the login all looks centered under 'Members Area' for me. Not sure if maybe you've already fixed that...
But if you put a background color on .login_sec that contains the login form you will see it is smaller than what you've put in it.
Like this -
Now remove the
height: 50px; and look at it again.
Request #2 is just a matter of making sure there is room for all the elements you want to float beside each other. Float them and, if needed, nudge them around with some margin to get them in the right spot
Request #3 might be a little more difficult since it's part of your menu and moving it out of your menu will likely remove most of the styling that makes it look the way it does. I did not look into moving this...
#4 is mabye not so difficult, just edit out the containing .header_innerright and all it's contents.