Ok to keep things short
A 250w setup is fine to keep anything and everything up to 24in deep.
MH reflectors cover a 2x2ft area. Lumenarc reflectors cover about 3x3. So on a 4ft tank you will need a double
DE fits a lil better into reflectors and gives a very slight better output. In turn they create more heat. Also they have no uv shielding, so a piece of normal glass of uv protecting acrylic must be used to not burn your retinas and sunburn everything in the tank.
Electric ballasts produce the absolute best efficiency, especially icecaps (which are all electric). Some magnetics produce more light, but at the expense of much more power used, and shorter bulb life. Also you must match pulse or probe start ballasts to the appropriate bulb, where most electric ballasts will fire everything.
A heatsink merely creates more surface area on the ballast for air to hit, therefore allowing more space for it to release the heat it creates.