H.p.s. Light/general ?

I have just bought a hps light of ebay with a ballast and everything for £66. It has a 400watt sylvania dual spectrum bulb. at the moment i am using 3 floro lights on two plants, and have six seeds germinating. 3 ice strains, and 3 skunk#1. when the seeds germinate, will they take kindly to the hps light or shall i put them with the floros, and then move them up to the hps.
Also, does anyone know how much bud i could gett of one plant if it was grown reasonably well with my light?
Medium i am using:
>1 part soil
>1 part perlite
>1 part gritty sand

Grown "reasonably well," I'd say a half an ounce. Really, really well? ... I'd say an ounce per plant.

Vi