A Radeon HD 5670 has a maximum board power of 64 Watts. This means it will draw up to 5.3 Amps from the +12 Volt rail through the PCI Express slot.
If the power supply has a minimum combined +12 Volt rails current rating of 22 Amps or more you will be safe.
i will go ahead and swap with my friends 305w PSU, which has a combined 12v value of 34a (to my knowledge, 12v + 12v2 = total?) that should do me well, unless im wrong about the combination...