1: 460w is enough as long as you are not overclocking.
2: dual slot means it will take up two of those silver backplate things on your case,not two pci e slot. You only need one of these.
you may want to consider buying a Radeon HD 6870 instead. They consume the same power,but the 6870 is a slightly more powerful. The 6870 is also about $10 cheaper
Hope I helped!