There might be some question as to which PSU is actually inside your case. 250W and 305W models may have been used in the 545s line. So check and find out which you actually have.
The Sapphire website recommends 400W PSU for its 4670s
(look under system requirements)
Same for the 5570 and 5450 (which seems wrong to me)
EVGA website recommends 300W PSU for it's GTS 240s