Newbie Qs: Sump, lights, cover (action!)

Hi. I'm planning a new marine aquarium and have a couple of simple
questions that I just haven't come across answers to. The basic plan is
for a 75 gallon system, 80 pounds live rock, live sand, protein skimmer,
powerheads, power filter, heaters, lights... just the basics, but enough
to develop a decent system. So here are a couple of remaining questions:

Sump: What do you do with it? I know the idea is that the sump allows
you to pull water down underneath the tank to have more water and do
"stuff" (technical term), but what does it allow you to avoid in terms of
equipment up above? It seems that heaters and powerheads will still have
to hang in the tank, won't they? Can you do protein skimming in the
sump? Does it substitute for the power filter? Or does it allow you to
add additional filtration (which a beginner might not need)?

Lights: I've read the various debates that wattage values don't tell you
everything about light needs in a tank (it depends on type of coral,
depth, etc.), but for a 75G tank, 3-5W/Gal suggests 225-375 watts. I was
looking at power compact retrofit kits, and there seem to be two options:
4 65W bulbs (setups around $275), or 4 96W bulbs (setups around $400).
The 4 65W give the minimum lighting, which I think would be fine for the
initial fish-only setup. But then here are some related questions:
- If I buy a setup with 4 65W, can I just substitute in some 96W
later? Or is there different ballast and other components for the bigger
bulbs? Are the 96W bulbs bigger? Can you mix bulbs, e.g. 65W and 96W?
I'd rather spend $100 more now if I have to replace the whole thing with
a $400 setup in a year (if PC is how I go).
- Is it ever a problem to have too much light?
- Why 48" and 46.5" bulbs? Just in case you have a tight-fitting
hood and need the 1 1/2" of space?

Acrylic/glass covers: Many of the finished light setups have acrylic
covers to protect the lights from seawater. Retrofit kits don't. I
assume that you still want some protection from water splashes even if
the lights are mounted a few inches above the water. Where do you get
covers, or do you?

Thanks in advance!
Scott