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Your suggestions for lighting please

I'm conceptualizing a new 75g (48 x 18 x 20) planted tank and puzzling a bit
over my lighting options. I think I'll use a Flourite or similar substrate,
supplemental CO2, and probably a cable heater. I envision a moderately
heavily planted tank with a good-sized fish load. I expect I'll want to keep
plant species with minimum light requirements that range from low to high.
I'll be using a hood, not an open top configuration.
I guess the rule of thumb usually tossed around for lighting is 2-4 watts
per gallon. If I split the difference and aimed for 3 watts/gallon that
would mean 225 watts. My first question is is that a reasonable figure?

Assuming 225 watts is my target I could get close to that with 4 55w compact
fluorescents. I could also do 2 96w CFs plus a 28w CF. Alternatively, using
VHO fluorescents, I could use 2 110 watt bulbs. With metal halides I'm not
sure. Two 175s would probably be too much. A single 250 watt bulb is in the
ballpark but I don't know if a single bulb will work well for a four foot
long tank.

So, those are the options I've thought of. Have I left anything out?

Now, the considerations. I suppose we have:

- initial cost
- cost of ownership (electricity usage, bulb life and replacement cost)
- heat from the bulb(s)
- heat from the ballasts(s)
- availability of suitable bulbs for planted aquaria
- form factor and adaptability to the hood and its dimensions
- aesthetics, i.e. point light from a metal halide vs. diffused light from
the fluorescents
- dimmability

Anything else you'd include? Your comments on all of these factors as they
relate to my various options would be greatly appreciated.

Concerning cost, I'm somewhat price sensitive but not overly so. If one
solution differs from another by 10 or 15% I'm not going to sweat it. If one
is twice the other, I'll give it some consideration.

Regarding aesthetics, I have noticed in some LFS displays that the metal
halide equipped tanks have a bit of shimmer that is appealing. That wouldn't
be an overriding factor for me though.

I'm intrigued by the idea of dimming because I can't imagine that it is all
that good for fish (maybe plants, too) to have to instantaneously transition
from darkness to full mid-day sun and vice versa. Is this of any practical
concern? As far as I know neither CFs or MHs are presently available in
dimmable forms. (With the exception of a particular Coralife MH unit that
uses a proprietary or special bulb - not really interested in that one).
Apparently, the IceCap VHO ballasts _are_ dimmable. I did read at a lighting
industry web site, however, that ordinary bulbs used in dimmable electronic
ballasts represent a fire hazard. I've not seen any mention in any of the
IceCap or Niche Engineering (makers of the Solar line of dimmers) literature
that any particular bulbs are required to take advantage of the dimming
feature. Can anyone shed any _light_ on this for me?

I guess that's it. Hopefully this will serve to spark a conversation that
will prove informative for others besides me.