I used that one power supply calculator ( http://extreme.outervision.com/PSUEngine ),and I got around 150 watts for my system (it dosent list the 4650,so I averaged a 4550 and 4670),so would it be safe to use a 200 watt PS?
You would probably be o.k, but likely it's pushing it. The power draw of the 4650 is closer to that of the 4670. The onboard video of the 9300 chipset isn't bad as it's more powerful than the 3200/3300 on AMD chipsets, so you may just want to use that instead.
I dont really want to get into too many details of my build, but my goal is to play Halo 2 Vista at higher settings, which the 4650 should do. Since its older game,do you think I could get away with using a lower powered GPU instead?