If you're able to change the fixtures, there are mercury discharge lamps, high output led street lamps such as
http://ledway-px.rtrk.com/LED-Products/LEDway-Street-Lighting.aspx

Thanks.
I was hoping for a more specific recommendation, like maybe the make and model of a good replacement fixture. The ones I've seen top out at 150W equivalent, but we use 300W lamps with medium beam width now, installed at ground level and aimed up to light the front of the church as well as the steeple.
I like the idea of LED lights, though they are still quite pricey but worth looking into. Also, I understand that mercury discharge lamps are being phased out like the T12 fluorescents.

I think you should talk to Rudd Lighting/ Cree or Color Kinetics/ Philips.

Do you have to point the lights up at the sky? This is pure light pollution. Is there any way you could point them down?

Even better, nothing is greener than just turning them off. :)

Here is a link that might be useful: International Dark-Sky Association (IDA)