Saltypimp, for a veg lamp that will cover a 5x5 area, you want at the MINIMUM a MH400 with the BEST bulb you have get (hortilux ftw!) and a parabolic type hood, but really you can go more watts than that. If you only run 400, it won't work with a crap bulb. Try a MH1000 for biggest oomph... heck, get a switchable K if you feel lucky
Remember that your flower watts should be about 3x the watts that would have taken to cover your vegging plants adequately. IF you veg under a switchable K, add another k or 2 of HPS when you go to flower. Vegging under a 400 and then flowering under a pair of 600s is a very nice ratio.