550 Watts is MORE than enough. You did not specify your CPU (note mox nixs on HDDs/DVD drive)
Typical i5-2500 system with a 6870 running furmark will draw around 350 Watts. You can find this in reviews.
My i5-2500k OCed to 4.2, 16 gigs ram (DDR3-1600 CL7 @1.60V), single 6870, 2 SSDs, 1 HDD, and 1 Blu-ray writer. Measured at the wall 350 Watts, since a typical PSU is only around 80->85% eff that means the the Load on the PSU is only around 280 W.
Reason Most GPUs over state PSU requirements is that there are a number of "Cheap"/junk PSUs out there that only delieve about 60-70 % of rate power once they warm up.
Added if You have the BP550 plus you have two Pci-e pwr connectors
1 x Main connector (20+4Pin)
1 x 12V(P4)
1 x 12V(8Pin)
6 x peripheral
6 x SATA
1 x Floppy
2 x PCI-E