Where to put ballast bags in a small boat

I am wanting to put about 400-600 pounds of ballast in my boat for when its just me and one other person. I have a small 1995 moomba boomerang (see profile) that has a maximum capacity rating for 5 people any more than 6 people in the boat and it is really sluggish. I was wondering would it be best to remove cushions and put 300-400Lb bag in the back and maybe 150 or so in the front?
or get? http://www.wakeside.com/page/w/PROD/wakeboard_boat_ballast/skylon_fat_buddies_hd_set_07

and put one on each side of the engine in the middle of the boat?
I have been going back and forth about what kind of weight to use and this seems like the best and easiest option.

http://www.wakeworld.com/MB/Discus/messages/65921/403794.html?1168829663

Heres some more pictures of my boat if it helps?

I'd take the back seat out and put about 600 back there. It worked well for me