You don't need a $120 750w psu for a single card. Or even a mid-range crossfire/sli setup (see my sig). For $300 (including psu), I'd be looking at a 6950 or 560ti.
I spent about 2 minutes searching, so you may be able to find better deals, but this will perform a lot better than a gts550. Don't get me wrong, the psu is very important, but there are quality units in the $50-60 range. There's no need to overspend and downgrade your gpu. And you can probably still crossfire 6950s on that power supply.