The MSI Radeon HD 5670 adheres to AMD's Radeon HD 5670 reference design. According to AMD the Radeon HD 5670's maximum board power is 65 Watts (5.3 Amps from the +12 Volt rail). The Radeon HD 5670 obtains all of its power through the PCIe slot so it doesn't require a PCIe supplementary power cable.
Since the SeaSonic SS-300TFX is able to supply up to 21 Amps on its +12 Volt rail you will have no problems using the Radeon HD 5670.