EDIT: Danny said it right, lol I read him wrong.
People often make the mistake of making billboards too bright and use less of them. You want more, dim, and angled billboards. It makes the edges of billboards less noticeable and more full of light, like it should be. Billboards are supposed to replicate fog within the light rays. Often, billboards don't cover the entire light area, even in the original game, and it makes me iffy =\ The key is to use more of them, angled, and darker. they'll even themselves out and give themselves the depth you;re looking for.
Example I searched on google:
Notice how the light rays (billboards) reach the edges of the light projected at the bottom? This is what you should aim for the best you can.
The billboards look fine in my opinion. Since this is a menu map, you could get away with actually sticking a large blue halo billboard (the spherical one used on candles and such) behind the rays to give it a bit more bloom and dramatic effect.