I'm still on my first grow, so take this with a grain of salt, but we've used one 250W HPS for veg and flower with decent results so far. day # 13 of flowering right now, and these pics go up until about a week ago.

http://www.flickr.com/photos/canuck-grower

we started with a lot of plants from seed and now we have 5 females left under that light. eventually we'll probably cut that down to 1-2 because of the smell. i'm happy enough with it for a first try, so I say go for it!