Haz beat me to it, it's creatine that retains water and gives you a bit more size and weight (due to water retention). I can't remember specifics but I know that supplimenting creatine helps with protein synthesis due to increased nitrogen stores within the muscles, as I said though I can't remember 100% so don't quote me on it.
Protien is very important though, I weigh 78kg at 5'10" and about 11% bodyfat and consume maybe 200g a day whether I've done my daily gym routine, had a day doing nothing, been riding trials or attending gymnastics. It helps to keep you feeling full and helps with the recovery of muscles.
bulkpowders is a great site to order from and it's usually at least half the price of highstreet shops (GNC charge around £35-50 for a 2kg tub depending on brand wheras Bulkpowers were about £15-20 last time I checked)