4x means 4 of a certain bulb. I'm running 4x 65W (half 10k, half actinic) + 2x 96W (2 50/50 bulbs) on a 55gallon. Going to replace the 4x65W fixture with 2 250W metal halides. Note that wattages aren't equivilent between fluorescents and metal halides (
MH being brighter).
Actinic refers to a purple/bluish bulb which can cause certain fish and corals to fluoresce. Its a very nice effect. If you wish to run normal output fluorescents (the 40W 4' tubes) I'd recommened an even mix of actinic and 10000K (refers to the color temperature, 10k being a 'crisp' white). If you have a capable fixture,
VHO gets you a lot more light (being a 110W 4' tube), but requires special equipment. I wouldn't run 96W
CF bulbs on a 55gallon tank. The bulbs are 36" long, which doesn't cover the 48" tank very well. 65W bulbs are 24" long and you can easily fit 4 of them over a tank. Note that
CF bulbs also require their own special fxture. You can't plug different kinds of fluorescents into another fixture and expect it to work.
There is a third class of lighting called T-5, which is also a fluorescent setup. Never tried it myself, but may be worth looking into.
Metal halides are a totaly different class of lighting than fluorescents. Instead of a long tube of phosphors, they are a smaller (and more expensive, bulbs run $65-$100 each) more pinpoint source of light. Expensive, run hot, but more or less required for some difficult to keep corals/clams/etc. Plus you get an awesome water shimmer effect.
My suggestion if you wish to keep corals, is a fixture resembling one of these:
PC lights:
http://hellolights.com/484xcoraqpch.html
http://www.marinedepot.com/aquarium...current-usa_orbit_sunpaq_fixtures.asp?CartId=
T5 lights:
http://www.marinedepot.com/aquarium_lighting_fluorescent_geisemann_basic_t5_60_system.asp?CartId=
Metal Halide:
http://www.marinedepot.com/aquarium...usa_outer_orbit_hqi_actinic_lunar.asp?CartId=
If you don't want to shell out the cash yet, literaly any lighting source will work for fish only. If you have a single strip light that can cover the aquarium, slap a bulb in it. Its not worth putting multiple Normal fluorescent fixtures over a tank as you'll be wasting money.