|When a manufacturer recommends a certain wattage power supply, it's for the entire system including the card, not for the card alone. Some manufacturers include an amperage requirement for the +12v rail as well. That's because majority of the power that goes to the video card comes from the +12v rail. But other components "feed" off the +12v as well. For example, the CPU get's 100% of it's power from the +12v, so if you have a CPU rated at 125W, it needs up to 10A. A gaming video card can easily require between 6-12A, depending on the card. HDDs, CD/DVD drives & all fans use the +12v too, so you can see that having adequate amperage +12v rail is critical to troublefree performance.|
You are NOT going to be able to run the HD4850 on a 300-350W PSU. I responded to one of your other threads & included a power consumption chart showing many modern video cards. Do NOT choose a card that takes the power supply close to it's limit. A system running a HD4850 is rated at 268W...that's a little to close for comfort with a 300W PSU.
BTW, you didn't have any problems with the 9400GT because it's a low end piece of sh!t, not a gaming card. It draws about 35W under full load (less than 3A on the +12v).