1 400 will do fine for a 55G, but I also recommend adding a powerhead on the end of the tank that will have poor circulation. I have 2 400's on my 55G, and it's great, good flow on both ends of the tank, and no need for a powerhead.
Or if you want to do something a little cheaper, a 400 on one end and a 280 on the other.
55G Filstar XP3 - 16" Fire Eel
75G Medium Planted - Filstar XP3 Low Light - Established Feb 2006