If your're stuck in 4:3, make sure you're not using the vesa driver, which doesn't know about 16:9 monitors. Run this command
and try to see
1. What your video card is (It will be some way down).
2. What driver the video card is using (Probably vesa)
3. Why it chooses whatever video mode it chooses.
Don't paste the whole thing pls - I won't read it. Post any useful snippets.
If you're not on vesa driver, there is a DisplaySize option you can put in xorg.conf.d/somefile.conf to set the size. it goes
DisplaySize xdim ydim - each in millimetres. Leave this until last thing to try.
Tell us how you got on.