5ftx5ftx7ft is an area of 25 cubic feet and a volume of 175 cubic feet .A 400 watt would not be suficient for an area of that size that works out at 16 watt per sft..
A 600 watt would be more efficient in a room that size. A 600 watt would give you 24 watts per sqft thats around the minimum wattage per sqft recomended which i think is 25 watts per sqft.The lumens from a 600 watt is around 92000 so that would give you 3680 lumens per sqft. to ventilate a room this size a inlet fan of a min of 60 cfm and and exaust fan of around 100 to110 cfm if carbon filter and duct installed

But better still a 1000 watt would give you better yeilds in an area of that size , i would recomend that you spend the extra cash(if you have it) on upgrading to 1000 watt interchangable ballast .A 1000 watt gives of around 140,000 lumens that would be 40 watts per sqft and 5600 lumens per sqft, this is an exellent light coverage for an area that size but cooling the area might become a problem an inlet fan with a cfm of around 80 should be efficient and an exuast fan of 160 cfm if using a carbon filter should do the trick.

Its all about prefrence really and what you can afford as i said the 400 watt would do if you arent planning on filling this space and only growing a couple of plants but if you plan filling it with plants then you want to do ur homework as if not problems will only arise later in the grow and beleave me and other growers will assure you of this that there is nothing worst that a grow going tits up mid flower because of poor preperatoin. i hope this helped . the postman


ps if you arent buying those lights from your mate give me a shout and i will have them been looking for a 400 watt mh for a project be safe the postman ukgwr