|You may need to look into the power supply's detailed specs. You old power supply might have used only one 12 volt pair of rails with a single amp rating, but the new power supply probably has two 12 volt rails each with it's own amp rating. If combined, the numbers add up to more amperage than the one you had before, but it is not used that way on your computer. Before, you might have had say 20 amps available that all circuits could draw from (numbers for example only, not accurate) and your graphics might have taken most of it. If now you have two 15 amp circuits, some circuits will be very happy with all they need, but the one that has the graphics card on it ONLY has 15 amps to share, graphics card, and all. You probably actually getting LESS available amps for graphics with the larger power supply.|
In short, look for a power supply with a single 12 volt rail around the 400 to 450 watt range, or go with a 500+ wattage with enough amps on each rail to cover your video card on one side (look for 12 volt amp specs on your old power supply).