Forget all that GARBAGE about that "generalized rule" of such and such gallons per hour. A canister filter is better at filtering than ANY HOB or simlar filter. The plus side to HOB's is the convenience of cleaning them. They do not force the water through the media like a canister does. Eheim is Top of the line, and has a lifetime reputation. It is well worth the investment. I have tried several different filters in my time with fish, and I have two different 55 gallons running right now. One has a Emperor 400 and a Whisper 300 (700gph nominal). This tank is my Blackbelt Cichlid tank (messy, large cichlid). My other 55 gallon is my high-light planted tank. This is currently running a Eheim ECCO 2233 canister and a Magnum HOT filter. It also uses pressurized CO2 which has a powerhead-driven reactor (Penguin 1140 - 300gph). I wish I could swap out the Mag HOT for another canister with slower flow. It is OK for now, but it does add quite a bit of current. This tank has a total of about 400gph and also the CO2 reactor adds another 300gph. This does have a prefilter on it so I consider it for filtration. I am even considering just using the ECCO and the CO2 reactor.
I would suggest (for most setups) to have 2 separate filters on a 4 foot tank. One might be able to do enough water turn-over, but it just can't keep enough water moving throughout the whole tank. This tends to cause water current dead-spots which accumulate wastes. I believe the ideal filtration for a 55 gallon (fully stocked, and able to handle about anything) is an Emperor 400 with a Eheim 2215 or Pro 2026. This would create beautifully clean water. You could also swap an AC500 for the EMP 400 if you desire less money, but great filtration.
