well, any good branded 450W or better will do for now. Your system is needing some upgrades such as your cpu and memory will fall behind with the second video card. Look into the future, and tell us how long you intend to have the power supply, this system getting more upgrades such as quad core?
Power supply for the future will have 4x and 8x cpu, and however many pci-e connectors you feel you need for video cards. Add in how much power you'll need which Anandtech had an article on. Bigger the power supply, the quieter it will be... technically.