1 metal halide every 2 feet is what most have recommended for MH.
Regarding 250w MH:
A 65 gallon tank is 24" tall, subtract the sandbed's depth and usually you get a 18-22" water column depth.
This said, the 250watt's are probably going to be a good maximum MH light for you. 250w MH is for "water columns" greater than 24" deep, up to +30" (again, it's what most have recommended).
I'm no MH expert, but a 400w MH for a 24" tall tank! :eek2: I guess to each their own, whatever works for you and your tank.
My 65 gal Megaflow tank has been up and running for 2 months now using 288watts of PC lighting and i'm upgrading to (2) 175watt MH 10k's and am supplementing with T5 aquablues. I am going with 2 MH's because I don't want the centerbrace shadow. I also figure that I can arrange the 2 MH's focal points to overlap, giving extra PAR in the center area of the tank for clams and strong light SPS's if it's really needed.
If you use a single MH over a 36" long tank... in order for the reflector to provide adequate coverage over the entire length of the tank... wouldn't you have to raise it 2-3 inches higher than usual? Thus taking some of the "oomph" out of the PAR?