550W doesnt surprise me. It'd be a good psu size with a bit of overhead. That said, I think ati says 450W with that gpu.
Its unfortunate you only want tiger as they dont have great selectionsd.
However, the cx500 is the best/inexpensive option, but its oly under 50 if you count the mir.
edit
If you look at a 500 zand a 550 of the same price, yo'll see the Amps on the 12V are the same. Basically they both put out the same power.
i'd suggest bumping your budget $15 and getting a better 500W.